Add support for struct constructors to SkSL.

This is allowed in OpenGL ES2, and its absence in SkSL has been a pain
point for new users adopting Runtime Effects.

Change-Id: Id2ed78261a2cd2b14b49ad22cb74cdc9e0905f8a
Bug: skia:11368
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/393418
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
22 files changed