add decal tilemode to shaders

Plenty more to follow-up:
- gradients
- gpu impl

Bug: skia:7638
Change-Id: I8e54fd0e24921f040f178c793b36c7fb855b136e
Reviewed-on: https://skia-review.googlesource.com/107420
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
15 files changed