This will facilitate intercepting the GL calls and implementing
emulation paths for offscreen redering. Or other EGL/GL features.
It does not yet override the GLES loader and uses the shared
'angle_util' loader as it did before.

Bug: angleproject:4845
Change-Id: I68806c307f59e1852999299e88dc358781817f12
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2305042
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>