Dynamic buffers are used to perform staged updates to
BufferVk objects instead of recreating staging buffers
for each update.

Bug: angleproject:4292
Change-Id: I0f64c821c97e0e6014e9df1d4f99e2f495838025
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2001461
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>