Allow dsl::Start with a Context instead of a Compiler.

This should allow lightweight DSL usage without needing access to a
Compiler, but some operations won't work (for now) because they rely on
the IRGenerator. Ideally, as we will reduce our dependence on
IRGenerator, these limitations will fade away.

Change-Id: I93880f153a7425b208bdc4a866169d037e4553a6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/451739
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
4 files changed