Add entry points and validation for various egl stream extensions
including EGL_KHR_stream_consumer_gltexture and
EGL_NV_stream_consumer_gltexture_yuv and NV_EGL_stream_consumer_external.
The extensions functionality is not yet implemented and the extension
strings are thus not exposed yet.

BUG=angleproject:1332
Change-Id: I115d872557db38d8dd94cc367038668406719109
Reviewed-on: https://chromium-review.googlesource.com/332026Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Ian Ewell <ewell@google.com>