Allow child effect passing in PipelineStageCodeGenerator.

In a followup, we can use this functionality to migrate the
KnownRuntimeEffects into the sksl_rt_shader module.

Bug: b/328051548
Change-Id: I39100b318f46834ee346f2bbc25375b1bb7a6523
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/881296
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
10 files changed