SkRuntimeEffect: Fix 'in' variables in CPU backend

We were never calling specialize() to bake in the values of ins,
so do that. Add uniformSize() to get the size of just the uniform
values. (The interpreter asserts that the size of the uniforms
being passed in matches the expected size from the ByteCode,
so these need to match up).

Added a unit test that uses both 'in' and 'uniform'.

Change-Id: I595822171211d35a17d5977fa790de0d1bbd6c78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/263519
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed