factor eval_13()/grad_13() for fewer multiplies

This oh-so-slightly tweaks the best fits.

Change-Id: I84ab8a47c2db16c992d1be696205b47e1d41294c
Reviewed-on: https://skia-review.googlesource.com/123100
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>
39 files changed