fiddle with perspective divide

Trade a divide for two multiplies.
This is almost always a good deal,
though Skylake's getting close.

Change-Id: Ic776145200957bb686edd00fbe0446212b436014
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299372
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed