xwayland: Fall back to GLES2 if we don't get at least GL 2.1 in glamor

Merged Adam Jackson requested to merge ajax/xserver:xwayland-glamor-gles2-fallback into master

Some particularly unfortunate hardware (Intel gen3, mostly) will give you GLES2 but not GL 2.1. Fall back to GLES2 for such cases so you still get accelerated GLX.

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