Previously, we would use a single offscreen framebuffer for both
transformations and when a shadow framebuffer should be used, but that
can be dreadfully slow when using software rendering with a discrete GPU
due to bandwidth limitations.
Keep the offscreen framebuffer for transformations only and add another
intermediate shadow framebuffer used as a copy of the onscreen
framebuffer.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/917
(cherry picked from commit 2b8b450fe16c21f0f37a1779560c0e5da61a9b89)
