Add BlendStateExt helper structure It provides compact storage and comparison operations for per-drawbuffer blend states. Added BitSetT::Zero() static constexpr. Bug: angleproject:4394 Change-Id: I66d6275facb7b28022fc24ff9cc0d8c3c976c99d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2154669 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>