Implement VarDeclaration and VariableReference in RP codegen.

We can now declare variables with values, and read those values back.

Change-Id: I05d4b484cea3c760fcf4fa655600881589265533
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/610639
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed