Refactor BlitGL to potentially hold multiple blit shader types. BUG=angleproject:1932 Change-Id: I4d19892852540797c765916131498c96511b4f55 Reviewed-on: https://chromium-review.googlesource.com/600509 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>