GL backend: add a workaround for bad default current vertex attributes BUG=angleproject:1492 BUG=351528 Change-Id: Ie4b25b0106282c9f60d19053e5a320549887bc8b Reviewed-on: https://chromium-review.googlesource.com/379196 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>