This will allow the TransformFeedback and other classes to share
the same buffer. Also should save a bit of memory.

Bug: chromium:1086532
Change-Id: I198170b4e09165a4770b68af6df9aa7b690e8d66
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2219138
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>