Daniel van Vugt authored May 24, 2018 and Marco Trevisan committed Jan 24, 2019

`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
by `clutter_actor_paint`.

Bonus optimization: This all means we don't need to keep `last_matrix_drawn`
or force a full repaint every time some part of the transformation change...