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-x11.c b/libweston/compositor-x11.c
index f9cb461..02cdf3e 100644
--- a/libweston/compositor-x11.c
+++ b/libweston/compositor-x11.c
@@ -389,7 +389,8 @@
 
 static int
 x11_output_repaint_gl(struct weston_output *output_base,
-		      pixman_region32_t *damage)
+		      pixman_region32_t *damage,
+		      void *repaint_data)
 {
 	struct x11_output *output = to_x11_output(output_base);
 	struct weston_compositor *ec = output->base.compositor;
@@ -457,7 +458,8 @@
 
 static int
 x11_output_repaint_shm(struct weston_output *output_base,
-		       pixman_region32_t *damage)
+		       pixman_region32_t *damage,
+		       void *repaint_data)
 {
 	struct x11_output *output = to_x11_output(output_base);
 	struct weston_compositor *ec = output->base.compositor;