sksl: Add builtinFMASupport to StandaloneShaderCaps

Bug: skia:
Change-Id: I8c32b09d3c4042707c4d6fc957c7bbff41022ce1
Reviewed-on: https://skia-review.googlesource.com/c/171524
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed