Only submit opaque (XRGB8888) buffers

This lets the compositor avoid needing to clear and blend the
pixels behind the background surface made by swaybg.

main.c

@@ -149,7 +149,7 @@ static void render_frame(struct swaybg_output *output, cairo_surface_t *surface)
 
 	struct pool_buffer buffer;
 	if (!create_buffer(&buffer, output->state->shm,
-			buffer_width, buffer_height, WL_SHM_FORMAT_ARGB8888)) {
+			buffer_width, buffer_height, WL_SHM_FORMAT_XRGB8888)) {
 		return;
 	}
 

pool-buffer.c

@@ -60,7 +60,7 @@ bool create_buffer(struct pool_buffer *buf, struct wl_shm *shm,
 	buf->size = size;
 	buf->data = data;
 	buf->surface = cairo_image_surface_create_for_data(data,
-			CAIRO_FORMAT_ARGB32, width, height, stride);
+			CAIRO_FORMAT_RGB24, width, height, stride);
 	buf->cairo = cairo_create(buf->surface);
 	return true;
 }