Add screenshot tests for output decorations
This MR depends on !1020 (merged).
Building on top of !1020 (merged) in order to capture not just output but also output decorations, this MR adds tests that ensure the decorations are drawn correctly. This brings three things that have never been tested under CI before:
- cairo-utils has code to draw window decorations, and now we verify at least some of that
- GL-renderer has code to blit cairo-utils decorations into output framebuffer, a.k.a output decorations
- color-lcms has code to create input-to-output color transformations
My primary goal is to get the input-to-output color transformations tested, which lead to the rabbit hole of everything else, because output decorations are pretty much the only feature using them.
This MR adds output decorations support in headless-backend, because wayland-backend which already has them would be more difficult to get set up. Then I add a non-color-managed test for output decorations, and a series of color-managed (i.e. set an output ICC profile) tests.