Make gl::DrawBufferMask a BitSet8 This shrinks the size of the mask for use in cache key classes. Bug: angleproject:4881 Change-Id: I87c234832c61e6a663c609b7f6da5d69977b21c7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2399182 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>