wayland: Add single pixel buffer support

The "single pixel buffer" Wayland protocol extension provides a way for
clients to create 1x1 buffers with a single color, specified by
providing the color channels (red, green and blue) as well as the
alpha channel as a 32 bit unsigned integer.

For now, this is turned into a 1x1 texture. Future potential
improvements is to hook things up to the scanout candidate logic and
turn it into a scanout capable DMA buffer.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2246>

src/wayland/meta-wayland-surface.c

@@ -780,8 +780,10 @@ meta_wayland_surface_apply_state (MetaWaylandSurface      *surface,
        * it until is replaced by a subsequent wl_surface.commit or when the
        * wl_surface is destroyed.
        */
-      surface->buffer_held = (state->buffer &&
-                              !wl_shm_buffer_get (state->buffer->resource));
+      surface->buffer_held =
+        (state->buffer &&
+         (state->buffer->type != META_WAYLAND_BUFFER_TYPE_SHM &&
+          state->buffer->type != META_WAYLAND_BUFFER_TYPE_SINGLE_PIXEL));
     }
 
   if (state->scale > 0)