This both generalizes the GL entry point enum to include other APIs
like EGL and inserts the EGL and WGL entry points into the enum.

This will faciliate EGL entry point auto-generation and also frame
capture for EGL entry points.

Bug: angleproject:2621
Change-Id: Iaf4310e03b3d55839dd1328362fb29dcef918fab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2555861
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>