sksl: Add a "sk_Clockwise" built-in

This allows us to identify clockwise-winding triangles, in terms of
Skia device space, in all backends and with all render target origins.

Bug: skia:
Change-Id: I220e1c459e0129d1cc4dee6458ef94277fbedd21
Reviewed-on: https://skia-review.googlesource.com/142662
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
13 files changed