Remove symmetric three stop special case

Just let the general three-stop shader handle all three-stop gradients.
Also, pre-compute values derived from the middle stop to remove all
division (and actually convert computation to FMA form).

Bug: skia:
Change-Id: I1aae069d929d1a942c38aa2e2f3fd5fb7d9b2f10
Reviewed-on: https://skia-review.googlesource.com/45800
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed