SkSL: Add support for method calls on effect objects

Includes .eval() as a new alternative syntax for sample/shade.

Bug: skia:12302
Change-Id: Ia921c141b3eeeba7e5309d921967b941f9cd055e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/444756
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
16 files changed