libGLESv2 builds with these changes on iOS, iOS Simulator, and
Mac Catalyst, using CGL or EAGL as appropriate. Tests will
require more work. Fixed several errors in the EAGL code which
hadn't been detected because the code wasn't built.

Bug: angleproject:4256
Bug: angleproject:5417
Change-Id: I29072d82607ef2500732c5cc00318fdab4a46b31
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2570211Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>