Add new SkVM trace op 'trace_scope' to track SkSL scope depth.

This CL adds the trace op and tests the builder commands, but it is not
yet hooked up to the SkVM code generator or debugger.

Change-Id: Iaa64293dfd0973e299eb480cb2b43672b0886c32
Bug: skia:12741
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/484439
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
8 files changed