Reland GrRectanizerPow2

This brings back the pow2 rectanizer for use with various ongoing
atlas experiments. If we can further optimize the skyline rectanizer,
then pow2 will be good to have around as a baseline comparison. And
if skyline gets fast enough, then we can delete pow2 again.

Change-Id: I79088c53fba7ba0d120534af99bee7840c135e42
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/290810
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
10 files changed