Add SkSL test for functions that operate on structs

This is a known deficiency of runtime effects, next step is to fix how
they manage function signatures to solve the problem.

Bug: skia:10939
Change-Id: Id934e0acdf774b03bd6edce78d7b2c077bdeae00
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/360603
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed