sksl: Support fwidth()

Adds fwidth() to SkSL, and adds a gm that draws an AA squircle to test
it.

Bug: skia:
Change-Id: Ida306cc535a1d4b4568d0ad5cc9a5f235098f4e8
Reviewed-on: https://skia-review.googlesource.com/c/170726
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
6 files changed