This is an experiment that adds texture diffing and exposes it via
GdkGLTextureBuilder::update-texture. It is built on top of !5862 (merged).
The code by itself looks fine to me, but I do not like exposing
cairo_region_t there. We could switch the API to use
graphene_rect_t instead, but then we couldn't expose regions.
Also, I added my testing commit if anyone wants to play with it, it produces output like this (with debug updates enabled, left is Cairo, right is GL): Screencast_from_2023-04-28_06-08-25