added sk_FragCoord support to skslc

BUG=skia:

Change-Id: If78a4d08121699f87659f0d2e35f3edbf1867401
Reviewed-on: https://skia-review.googlesource.com/5408
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
33 files changed