Add support for indirect trace_var in SkRP codegen.

Change-Id: Ice1d1829d8a752863449e0f99388278467c7e908
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/663596
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
9 files changed