fix tricolor shader

1. map pixel-centers through inverse (correctness)
2. forward-difference srcXY in loop (perf)

Lots of minor (but correct) changes in GMs

Bug: skia:6578
Change-Id: I2ad2ef939f58373d33212d4056c7d2f63eaafe0e
Reviewed-on: https://skia-review.googlesource.com/15153
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed