clutter: Fix offscreen-effect painting of clones
ClutterOffscreenEffect had been getting the wrong bounding box in the
case of clones and descendents of clones, causing visibly incorrect
clipping. This was due to
clutter_actor_get_paint_box only ever being
given the source actor during a paint (which is correct) and not the clone.
Even if we weren't painting a clone but an offscreened descendent of a
clone (like in gnome-shell's desktop zoom), we would get the wrong result.
Fortunately we don't need to know the actual clone/actor being painted so
don't need to call the problematic
clutter_actor_get_paint_box at all.
The solution is to only keep untransformed rendering in the FBO and leave
the correct transformation for later. The correct clone/actor's
transformation is already set for us as the current cogl modelview matrix
Bonus optimization: This all means we don't need to keep
or force a full repaint every time some part of the transformation changes.
Because the FBO contents are no longer affected by transformations. As it
should be. In other words, offscreen-effected actors can now move around
on screen without themselves being repainted.
Special thanks to Mai Lavelle for identifying the cause of the problem.
Closes: https://bugzilla.gnome.org/show_bug.cgi?id=789050, https://bugzilla.gnome.org/show_bug.cgi?id=659523#c9, #196, #282, gnome-shell#387, https://launchpad.net/bugs/1767648, https://launchpad.net/bugs/1779615