compositor: clarify update_transform vs. repaint

Remove redundant weston_surface_update_transform() calls from within
output repaint paths, and add a comment that we need to rely on
surface->geometry.dirty == 0 within the repaint sub-functions.

Now that weston_surface_update_transform() does damage as needed, and
weston_output_repaint() explicitly calls update_transform, we can reduce
the updates in rotate_grab_motion() to simply scheduling a repaint. This
will guarantee that the change in rotation ends up on screen ASAP.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/src/shell.c b/src/shell.c
index 5632c73..fbb1203 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -1034,6 +1034,11 @@
 		weston_matrix_init(&surface->rotation.rotation);
 		weston_matrix_init(&rotate->rotation);
 	}
+
+	/* Repaint implies weston_surface_update_transform(), which
+	 * lazily applies the damage due to rotation update.
+	 */
+	weston_compositor_schedule_repaint(surface->surface->compositor);
 }
 
 static void