wayland: Move initialization to the Rust side

This will help make the init procedure safer, by limiting the number of Rust objects that need to be carried to the C side and may be mangled on the way there.

The second benefit is that it allows outputs to become part of new state management.

src/wayland.c

@@ -4,6 +4,12 @@
 
 struct squeek_wayland *squeek_wayland = NULL;
 
+void squeek_wayland_init_global(struct squeek_outputs *outputs) {
+    struct squeek_wayland *wayland = {0};
+    wayland->outputs = outputs;
+    squeek_wayland = wayland;
+}
+
 // The following functions only exist
 // to create linkable symbols out of inline functions,
 // because those are not directly callable from Rust