When allocations are made from DynamicBuffer, they suballocate from a
possibly larger BufferHelper.  In BufferVk, the offset of the
suballocation was discarded, which limited the use of DynamicBuffer to a
pool of small buffers.

This change applies any such offset that may arise from suballocations
everywhere, and makes BufferVk use a larger buffer size when the
GL_DYNAMIC_* buffer usage hints are provided.

Bug: angleproject:5719
Change-Id: I3df3317f7acff1b1b06a5e3e2bb707616a7d0512
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2738650
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>