Use SkSTArenaAlloc to manage SkCoverageDeltaMask's stack memory

This way, we could have more stack memory on Google3: if each of the
two branche has 12K stack memory, Google3 would believe that it needs 24K
stack memory; but using SkSTArenaAlloc, we could use 12K stack memory to
handle those two branches.

Bug: skia:
Change-Id: Ie9234226cd4ba93b5be2ebeb95ab771031354f97
Reviewed-on: https://skia-review.googlesource.com/42101
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
4 files changed