(platform) adding GLES2/3 support back

Platform/egl-gles is still somewhat broken,
but is next up for being fixed. We will likely
need a platform for egl-nvidia and all their
specific extensions for display probing etc.

Also fixes odd regression worth noting:
For "blank" frames (nothing updated, no-reason
to redraw the RT), egl-dri crashes on swapBuffers.