x11: Make a sacrificial glX context to check for extensions during init. This is necessary because we need to see if GLES compat extensions exist. All of this code (including ShouldUseTextureFramebuffer()) should be revisited after 2.0.6 ships; ideally we don't make throwaway contexts if we can avoid it...but maybe we can't. I hear Vulkan is pretty cool. Fixes Bugzilla #3725.