Add software GL-renderer support to the headless backend
Submitted by Pekka Paalanen
Assigned to Wayland bug list
Similar to bug #83983, but hook up the GL-renderer, and select it with renderer=software-gl.
If possible from within the headless backend, force Mesa to run llvmpipe. It's probably not possible, so some additions to the test scripts are needed. Mesa's EGL null platform might be useful for this. Actual rendering could be done to an FBO if needed.
If EGL null platform init fails, the implementation is not Mesa, or whatever that implies the resident GL stack is not capable of running this test, the test needs to be skipped, not failed.
Add a test, that excercises the GL-renderer; just start Weston on headless with GL, map a window, wait for a frame callback, and exit.