Added modifier support to DSLFunction

This also includes a couple of fixes to DSLFunction error reporting.

Change-Id: Iab813143511c408df2dba7a4dc8beddea1a8e8d0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/427736
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
5 files changed