Allow backends to group repaint flushes

Implement new repaint_begin and repaint_flush hooks inside
weston_backend, allowing backends to gang together repaints which
trigger at the same time.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/libweston/compositor-headless.c b/libweston/compositor-headless.c
index a1aec6d..9e42e7f 100644
--- a/libweston/compositor-headless.c
+++ b/libweston/compositor-headless.c
@@ -92,7 +92,8 @@
 
 static int
 headless_output_repaint(struct weston_output *output_base,
-		       pixman_region32_t *damage)
+		       pixman_region32_t *damage,
+		       void *repaint_data)
 {
 	struct headless_output *output = to_headless_output(output_base);
 	struct weston_compositor *ec = output->base.compositor;