Use headless context instead of OsMesa

[jdm]

Glutin's context builder supports a build_headless API. We should see if we can use that, since it will get us headless support on all platforms rather than the current OsMesa solution.

[SimonSapin]

As far as I can tell, on Linux the code behind this build_headless API relies on the presence of a X11 or Wayland server (only without creating a window). So I suspect it requires a hardware GPU, whereas OSMesa provides a CPU-based/software implementation of GL.

(Glutin also has glutin::os::unix::OsMesaContextExt, which might help simplify our code a bit but does not otherwise change much.)

[SimonSapin]

OSMesa-src and Glutin is are longer used since #25853.