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-fbdev.c b/libweston/compositor-fbdev.c
index 6f976d1..32d71e0 100644
--- a/libweston/compositor-fbdev.c
+++ b/libweston/compositor-fbdev.c
@@ -118,7 +118,8 @@
 }
 
 static int
-fbdev_output_repaint(struct weston_output *base, pixman_region32_t *damage)
+fbdev_output_repaint(struct weston_output *base, pixman_region32_t *damage,
+		     void *repaint_data)
 {
 	struct fbdev_output *output = to_fbdev_output(base);
 	struct weston_compositor *ec = output->base.compositor;