GL: track enabled vertex arrays as a count rather than a mask

Bug: skia:
Change-Id: I63e70ab844a7e04df20165aba025b963efcafa9e
Reviewed-on: https://skia-review.googlesource.com/15630
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
3 files changed