Improve key rendering and eliminate two deprecation warnings

- Replace two calls to deprecated gdk_cairo_create()

- Alleviate asymmetry between rendering pressed and released keys
  by consistenly clipping the same area up front to avoid
  artefacts

- make sure pressed and released keys are shown immediately by
  calling gdk_window_invalidate_rect()

- improve consistency between render_(pressed|locked|released)_key

- improve code flow

eek/eek-gtk-keyboard.c

@@ -31,6 +31,7 @@
 #include <canberra-gtk.h>
 #endif
 
+#include <math.h>
 #include <string.h>
 
 #include "eek-gtk-keyboard.h"
@@ -479,6 +480,45 @@ magnify_bounds (GtkWidget *self,
     large_bounds->y = CLAMP(y, 0, allocation.height - large_bounds->height);
 }
 
+/*
+ * Alleviate the asymmetry between drawing a pressed key and a released key,
+ * and consistently draw to the exact same area.
+ *
+ * By saving the dirty rectangle we can limit drawing of the backbuffer to
+ * the screen as well, eg gdk_window_invalidate_rect() instead of
+ * gtk_widget_queue_draw() which redraws the entire widget.
+ *
+ * b1 is mandatory, b2 is optional
+ */
+static GdkRectangle
+clip_bounds_to_dirty_rectangle (cairo_t *cr, EekBounds *b1, EekBounds *b2)
+{
+    if (b2)
+        cairo_rectangle (cr, b2->x, b2->y, b2->width, b2->height);
+
+    cairo_rectangle (cr, b1->x, b1->y, b1->width, b1->height);
+    cairo_clip (cr);
+
+    /*
+     * save the clipped region to a bounding box so we can limit
+     * the drawing of the backbuffer to the screen to the same area
+     */
+    cairo_rectangle_t bbox;
+
+    cairo_clip_extents (cr, &bbox.x, &bbox.y, &bbox.width, &bbox.height);
+
+    /* convert double to int, making sure r strictly covers bbox to avoid
+     * artefacts. floor() is unnecessary, ceil() is not */
+    GdkRectangle r = {
+        floor (bbox.x),
+        floor (bbox.y),
+        ceil  (bbox.width),
+        ceil  (bbox.height)
+    };
+
+    return r;
+}
+
 static void
 render_pressed_key (GtkWidget *widget,
                     EekKey    *key)
@@ -486,16 +526,20 @@ render_pressed_key (GtkWidget *widget,
     EekGtkKeyboardPrivate *priv = EEK_GTK_KEYBOARD_GET_PRIVATE(widget);
     EekBounds bounds, large_bounds;
 
+    GdkWindow         *window  = gtk_widget_get_window (widget);
+    cairo_region_t    *region  = gdk_window_get_clip_region (window);
+    GdkDrawingContext *context = gdk_window_begin_draw_frame (window, region);
+    cairo_t           *cr      = gdk_drawing_context_get_cairo_context (context);
+
     eek_renderer_get_key_bounds (priv->renderer, key, &bounds, TRUE);
     magnify_bounds (widget, &bounds, &large_bounds, 1.5);
 
-    GdkWindow *window = GDK_DRAWABLE (gtk_widget_get_window (widget));
+    /*
-    cairo_region_t *region = gdk_window_get_clip_region (window);
+     * clip to limit drawing to backbuffer and save clip region to dirty_rect
+     * to limit redrawing of the backbuffer to the same area
+     */
+    GdkRectangle dirty_rect = clip_bounds_to_dirty_rectangle (cr, &bounds, &large_bounds);
 
-    GdkDrawingContext *context = gdk_window_begin_draw_frame(
-        window, region
-    );
-    cairo_t *cr = gdk_drawing_context_get_cairo_context(context);
     cairo_save (cr);
     cairo_translate (cr, bounds.x, bounds.y);
     eek_renderer_render_key (priv->renderer, cr, key, 1.0, TRUE);
@@ -506,8 +550,12 @@ render_pressed_key (GtkWidget *widget,
     eek_renderer_render_key (priv->renderer, cr, key, 1.5, TRUE);
     cairo_restore (cr);
 
-    gdk_window_end_draw_frame(window, context);
+    gdk_window_end_draw_frame (window, context);
-    cairo_region_destroy(region);
+
+    cairo_region_destroy (region);
+
+    /* force immediate drawing of the backbuffer to the screen */
+    gdk_window_invalidate_rect (window, &dirty_rect, FALSE);
 }
 
 static void
@@ -516,15 +564,29 @@ render_locked_key (GtkWidget *widget,
 {
     EekGtkKeyboardPrivate *priv = EEK_GTK_KEYBOARD_GET_PRIVATE(widget);
     EekBounds bounds;
-    cairo_t *cr;
 
-    cr = gdk_cairo_create (GDK_DRAWABLE (gtk_widget_get_window (widget)));
+    GdkWindow         *window  = gtk_widget_get_window (widget);
+    cairo_region_t    *region  = gdk_window_get_clip_region (window);
+    GdkDrawingContext *context = gdk_window_begin_draw_frame (window, region);
+    cairo_t           *cr      = gdk_drawing_context_get_cairo_context (context);
 
     eek_renderer_get_key_bounds (priv->renderer, key, &bounds, TRUE);
+
+    /*
+     * clip to limit drawing to backbuffer and save clip region to dirty_rect
+     * to limit redrawing of the backbuffer to the same area
+     */
+    GdkRectangle dirty_rect = clip_bounds_to_dirty_rectangle (cr, &bounds, NULL);
+
     cairo_translate (cr, bounds.x, bounds.y);
     eek_renderer_render_key (priv->renderer, cr, key, 1.0, TRUE);
 
-    cairo_destroy (cr);
+    gdk_window_end_draw_frame (window, context);
+
+    cairo_region_destroy (region);
+
+    /* force immediate drawing of the backbuffer to the screen */
+    gdk_window_invalidate_rect (window, &dirty_rect, FALSE);
 }
 
 static void
@@ -533,26 +595,29 @@ render_released_key (GtkWidget *widget,
 {
     EekGtkKeyboardPrivate *priv = EEK_GTK_KEYBOARD_GET_PRIVATE(widget);
     EekBounds bounds, large_bounds;
-    cairo_t *cr;
 
-    cr = gdk_cairo_create (GDK_DRAWABLE (gtk_widget_get_window (widget)));
+    GdkWindow         *window  = gtk_widget_get_window (widget);
+    cairo_region_t    *region  = gdk_window_get_clip_region (window);
+    GdkDrawingContext *context = gdk_window_begin_draw_frame (window, region);
+    cairo_t           *cr      = gdk_drawing_context_get_cairo_context (context);
 
     eek_renderer_get_key_bounds (priv->renderer, key, &bounds, TRUE);
-    magnify_bounds (widget, &bounds, &large_bounds, 2.0);
+    magnify_bounds (widget, &bounds, &large_bounds, 1.5);
-    cairo_rectangle (cr,
+
-                     large_bounds.x,
+    /*
-                     large_bounds.y,
+     * clip to limit drawing to backbuffer and save clip region to dirty_rect
-                     large_bounds.width,
+     * to limit redrawing of the backbuffer to the same area
-                     large_bounds.height);
+     */
-    cairo_rectangle (cr,
+    GdkRectangle dirty_rect = clip_bounds_to_dirty_rectangle(cr, &bounds, &large_bounds);
-                     bounds.x,
-                     bounds.y,
-                     bounds.width,
-                     bounds.height);
-    cairo_clip (cr);
 
     eek_renderer_render_keyboard (priv->renderer, cr);
-    cairo_destroy (cr);
+
+    gdk_window_end_draw_frame (window, context);
+
+    cairo_region_destroy (region);
+
+    /* force immediate drawing of the backbuffer to the screen */
+    gdk_window_invalidate_rect (window, &dirty_rect, FALSE);
 }
 
 static void