macos: various OpenGL performance fixes

few slightly unrelated but useful fixes as well

this gets me to 60fps when scrolling text in gtk4-demo as well as (after macOS finally decides to up the GPU package energy) gtk4-widget-factory too.

note the changes to GSK which make a bunch of things const to lower the cost of calling all the getters from the renderers.

one could argue that maybe we instead just make all of those structures live in the private.h and avoid use of getters from renderers.