Add support for creating EGL pixmaps from X11 pixmaps using GLX.
Pixmaps are needed for various external APIs such as VAAPI.

Add support for EGL_NOK_texture_from_pixmap to allow binding
pixmaps to textures.

BUG=angleproject:4560

Change-Id: I4a6d3ad7e87151ff5317bbdaaf093ac1b46daf5f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2153805
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>