weston_buffer: Add solid buffer type

Currently solid-colour displays (e.g. the background for fullscreen
views) is implemented by a special-case weston_surface which has no
buffer attached, with a special punch-through renderer callback to set
the colour.

Replace this with a weston_buffer type explicitly specifying the solid
colour, which helps us eliminate yet more special cases in renderers and
backends.

This is not handled yet in any renderer or backend, however it is also
not used by anything yet. Following commits add support to the renderers
and backends.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/libweston.h

@@ -1181,12 +1181,16 @@ struct weston_buffer {
 		WESTON_BUFFER_SHM,
 		WESTON_BUFFER_DMABUF,
 		WESTON_BUFFER_RENDERER_OPAQUE,
+		WESTON_BUFFER_SOLID,
 	} type;
 
 	union {
 		struct wl_shm_buffer *shm_buffer;
 		void *dmabuf;
 		void *legacy_buffer;
+		struct {
+			float r, g, b, a;
+		} solid;
 	};
 
 	int32_t width, height;
@@ -1768,6 +1772,13 @@ weston_surface_create(struct weston_compositor *compositor);
 struct weston_view *
 weston_view_create(struct weston_surface *surface);
 
+struct weston_buffer_reference *
+weston_buffer_create_solid_rgba(struct weston_compositor *compositor,
+				float r, float g, float b, float a);
+
+void
+weston_buffer_destroy_solid(struct weston_buffer_reference *buffer_ref);
+
 void
 weston_view_destroy(struct weston_view *view);