Implement statements and expressions in DSL C++ code generator.

This CL removes the bulk of the existing C++ code generator, especially
all the complex format-string assembly code. It has been replaced with
actual DSL code generation. Simple IR can now be successfully translated
to a working DSL fragment processor.

This CL also adds a simple test harness which is patterned after the
existing SkSLTest; it renders a pixel, reads it back, and fails the test
if the result isn't solid green (RGBA=0101).

This CL doesn't implement every feature. Some obvious gaps include:
- Sampling from children
- Uniforms/inputs of any kind
- Function calls of any kind

Change-Id: Ib80c23fe1ba4453f7c3cb43b65f93c5ea0deb709
Bug: skia:11854
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396757
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
37 files changed