ActiveTextureMask and ImageUnitMask need more than 64bits. We
can now use the BitSetArray datatype and avoid the perf penalty
of using std::bitset

Bug: angleproject:3877
Tests: angle_perftests.exe --gtest_filter=TexturesBenchmark*
Change-Id: Ic82e09d1aa2c1904d06ad48ba6200f35896665ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2664734
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>