Expose the Input variable and Child name collections in SkRuntimeEffect

Add framework for unit tests that draw (CPU and GPU) with a runtime
shader, as well as couple example tests.

Change-Id: I43b3b39e86634ec55521a2689a4c55c21939dce5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/262809
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed