Headless rendering doesn't work on windows

[jdm]

Right after setting up a GL object, we invoke these GL methods. However, our get_proc_address implementation on windows is a stub so this immediately backfires.

[SimonSapin]

Our headless rendering is based on OS Mesa, but it looks like we only compile it on Linux and OS X. @emilio, do you know why?

[emilio]

I think that's just because we only run reftests on those platforms IIRC

[jdm]

The build script is based on unix tools, so it's not a straightforward fix. It's possible that the updated mesa that relies on meson instead of autotools might be easier.

[jdm]

Another option might be to build a Windows-specific headless mode that's based on ANGLE (which just merged in #22856) somehow. https://github.com/stackgl/headless-gl/blob/master/src/webgl.cc might be a usable model.