weston_buffer: Add type field

Rather than open-coding various resource -> type accessors, just stick a
type enum in the buffer struct.

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

include/libweston/libweston.h

@@ -1177,10 +1177,18 @@ struct weston_buffer {
 	struct wl_signal destroy_signal;
 	struct wl_listener destroy_listener;
 
+	enum {
+		WESTON_BUFFER_SHM,
+		WESTON_BUFFER_DMABUF,
+		WESTON_BUFFER_RENDERER_OPAQUE,
+	} type;
+
 	union {
 		struct wl_shm_buffer *shm_buffer;
+		void *dmabuf;
 		void *legacy_buffer;
 	};
+
 	int32_t width, height;
 	uint32_t busy_count;
 	int y_inverted;