This allows the app to update Buffer data while the data is in use
by the GPU. For instance, uploading new vertex attribute data after
a draw call.

It introduces a StagingBuffer helper class, similar to StagingImage.
These classes are somewhat temporary and could be redesigned.

BUG=angleproject:2200

Change-Id: If8634b1411779b16c2bd22cce18a5f37ed958d1c
Reviewed-on: https://chromium-review.googlesource.com/756959Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>