Prefer EGL to GLX for the GL support on X11

GLX is considered legacy, at this point. New functionality to expose zero-copy buffer passing is implemented as EGL extensions, for instance. The Mesa developers have recommended using EGL in place of GLX for a while, now.

The Wayland and Windows backend already have switched to EGL, so might as well bring the X11 backend forward.

The GLX backend support is still in place in case EGL is not available—and as a debug toggle.