Add GrRuntimeFPBuilder

Like SkRuntimeShaderBuilder but for internal use for creating FPs.

Currently it requires that the code be a static string so it can
easily make a static SkRuntimeEffect instance for each effect.

Bug: skia:11771

Change-Id: I18148eb33e7d28c804e4a13bcef88c89c06b2c9b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/386889
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
11 files changed