This adds support for the following functions:

- glDrawElementsBaseVertex
- glDrawRangeElementsBaseVertex
- glDrawElementsInstancedBaseVertex

Bug: angleproject:3582
Bug: angleproject:3402
Bug: angleproject:4166
Change-Id: I83770f62e3a918c0965fd4ca8c7d9e598b8b4154
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1929083
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>