Cache maximum enabled vertex attribute. This can give us much faster draw call validation, by saving us from checking buffer sizes for non-enabled attribs. Also for checking if vertex buffers are mapped. Gives >100% increase in the benchmark. BUG=angleproject:959 Change-Id: I211c310385bdee46ed06f68ecd9c98385e1f8db9 Reviewed-on: https://chromium-review.googlesource.com/267751 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org>