We were using a src offset instead of a dest offset. The staging
buffer should always start at the zero offset, while the dest
buffer (the real buffer) should use the offset argument to
Buffer*Data.
Bug: angleproject:2374
Change-Id: I9ee702709d9c15945935f4eae25f308b71e0d1e8
Reviewed-on: https://chromium-review.googlesource.com/937022
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
