Little clean-ups on pixman, headless, wayland, x11

While implementing output resizing API for renderers so that I could implement the new screenshooting protocol so that I could take screenshots of output decorations so that I could exercise CMLCMS_CATEGORY_INPUT_TO_OUTPUT type of color transformations in CI, I decided to pick the simple preparation patches I had so far and get them landed to reduce the number of patches to follow. Here they are, a set of various little bits that are hopefully trivial.