VertexArray: Add enabled attribs bitmask. 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/847284 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>