Refactor and share the output decorations GL-renderer setup code between headless and wayland backends

Headless-backend carrying a copy of a piece of wayland-backend code that itself is a 4x copy-paste is awkward to maintain. Now that the code in headless-backend is covered by CI testing, it's time to refactor and consolidate it.

The series starts with an off-topic meson patch. Then the 4x copy-paste code is re-written as a loop, added a little clean-up fix, converted into backend-agnostic code, moved into a neutral place, and finally wayland-backend drops all the equivalent code to use the shared code.

Headless-backend is tested by CI, but wayland-backend is not. I manually checked that wayland-backend decorations look and work fine, but I don't know how to test output resizing.