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>