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
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 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.

Fixes:
https://bugzilla.gnome.org/show_bug.cgi?id=789050,
https://bugzilla.gnome.org/show_bug.cgi?id=659523#c9,
GNOME/mutter#196,
GNOME/mutter#282,
GNOME/gnome-shell#387,
https://launchpad.net/bugs/1767648,
https://launchpad.net/bugs/1779615
2 jobs for fix-offscreen-clones-v1 in 5 minutes and 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #195789
build-mutter

00:01:29

 
  Test
passed #195790
test-mutter

00:03:33