compositor: introduce weston_surface_geometry_dirty()

Instead of directly setting the dirty flag on weston_surface geometry,
use a function for that.

This allows us to hook into geometry dirtying in a following patch.

Also add comments to weston_surface fields, whose modification causes
transform state to become outdated.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/src/animation.c b/src/animation.c
index 9e2ad4e..229946f 100644
--- a/src/animation.c
+++ b/src/animation.c
@@ -116,7 +116,7 @@
 	wl_list_remove(&animation->animation.link);
 	wl_list_remove(&animation->listener.link);
 	wl_list_remove(&animation->transform.link);
-	animation->surface->geometry.dirty = 1;
+	weston_surface_geometry_dirty(animation->surface);
 	if (animation->done)
 		animation->done(animation, animation->data);
 	free(animation);
@@ -153,7 +153,7 @@
 	if (animation->frame)
 		animation->frame(animation);
 
-	animation->surface->geometry.dirty = 1;
+	weston_surface_geometry_dirty(animation->surface);
 	weston_compositor_schedule_repaint(animation->surface->compositor);
 }