re-land of added sk_FragCoord support to skslc

BUG=skia:

Change-Id: Ifac1aa39839058787ad1794200c3dbb93c147a69
Reviewed-on: https://skia-review.googlesource.com/5850
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
33 files changed