This is a refactor-only change mostly. It makes the classes work
more similarly to how the other classes in the Vk back-end work. Also
removes some redundant code. Moves the buffer caching into begin for a
more dirty-bits like approach.
Bug: angleproject:4622
Change-Id: I34ac13e1d05b48e3267937c760d195493a76ed02
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2191172
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
