Create SkSL::RP::Builder class.

This gives us a way to assemble an RP-based program in a convenient
and testable way. At present, there are only a handful of supported
opcodes; additional opcodes will be added as the code generator
evolves.

Change-Id: Idd389a2d0ba8fc4c712e36fdd8aae7dacecd3caf
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/603403
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed