Add the entrypoint, and when a buffer is immutable, recreate it
with glBufferStorageEXT.

Test: MEC for Dota Underlords
Bug: b/185192780
Bug: angleproject:5857
Change-Id: I29b934bdff4185d19594e49e9387dd79c6031aa5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2821945Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>