compositor: clarify update_transform vs. repaint

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>