- Remove level parameter from GetRenderbufferImage.
- Add packed enum handling to GetTexImage.
- Fix ext spec reference to table 8.4.

Bug: angleproject:3944
Change-Id: I8f67608396217aaddb789dd95d0f3e2378ce918d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1879960Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>