Provide a surfaceless EGL headless backend

[brunoabinader]

This recent Mesa extension - MESA_platform_surfaceless - provides a native display independent of any native window system. Given this solution already works and is testable via CI, we can pursuit this as a follow-up work.

My initial tests on this were not successful - I've been building mesa with EGL surfaceless support - see https://github.com/mapbox/mason/tree/mesa-13.0.0-egl-surfaceless - and using the example usage from the specification author but so far no luck in initializing an EGL display.

Follow-up from #5843 and #7007.

/cc @mikemorris @kkaefer

[jfirebaugh]

Current implementation suffices and is stable; this doesn't seem necessary.