Vulkan: Support the indices offset in drawElements calls Also fixes an issue in buffer copy synchronization. Bug: angleproject:2645 Change-Id: Ibca7052daaf1e6fe37913c8a8216ec33c66426b6 Reviewed-on: https://chromium-review.googlesource.com/1096911 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Luc Ferron <lucferron@chromium.org>