This replaces the "max enabled attrib" integer with a bitmask of
enabled attribs. Should have better worst-case performance (only
attribute 15 is enabled) and similar best-case performance (when
only attribute 0 is enabled).

This might also help implementing validation optimizations.

Bug: angleproject:2202
Change-Id: I5cbb533c3af23851a42c80a6dc409a0da84e87c3
Reviewed-on: https://chromium-review.googlesource.com/847284Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>