This steps towards the goal of auto-generating EGL entry points.
Also cleans up some EP definition inconsistencies.

Bug: angleproject:2621
Change-Id: I94dcf3d080b6864bca3f9db6c4c01429f4f8b91f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2553968
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>