Vulkan: Clamp vertex buffer offsets. Vulkan mandates that vertex buffer offsets are within the size of the buffer even if robust buffer access is enabled. Do a check in VertexArrayVk::syncDirtyAttrib to guard against this. Bug: angleproject:3350 Change-Id: Ia01deb9b7335dd8f632f2cf9b32d2908ba55645c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2044491 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>