Vulkan: Fix offsets in BufferVk::setDataImpl. 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>