Support structs in runtime effects

Uses the pipeline-stage callback mechanism. It mangles the type name
(with a test to verify that this works), and then calls defineStruct
with the entire SkSL struct definition string.

Bug: skia:10939
Change-Id: If14cf1b11faaa80ad8d4086cdacf68532bac43fc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368809
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
17 files changed