glamor_egl: Reject OpenGL < 2.1 early on

Merged Lubomir Rintel requested to merge lkundrak/xserver:lr/modesetting-gles2 into master

The Etnaviv driver on GC2000 reports desktop OpenGL 1.3 but also OpenGL ES 2.0. However, with the modesetting driver, GLES2 never gets a chance:

  [ 11233.393] Require OpenGL version 2.1 or later.
  [ 11233.393] (EE) modeset(0): Failed to initialize glamor at ScreenInit() time.
  [ 11233.393] (EE)
  Fatal server error:
  [ 11233.395] (EE) AddScreen/ScreenInit failed for driver 0

Let's reject old desktop GL early on, just like XWayland seems to do.

This is perhaps a slightly bit more complicated that one would expect, since we need to call eglMakeCurrent() before we query the GL version.

Signed-off-by: Lubomir Rintel

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