Add runtime color filter and shader modes to the SkSL compiler

These enforce stricter rules about the signature of main, and each one
uses a separate pre-include module. That prevents color filters from
being able to reference sk_FragCoord (or coords passed to main) at all.
It also limits the versions of sample() that are exposed.

In the new world, an effect created for a specific stage of the Skia
pipeline can only be used to create instances of that stage (SkShader or
SkColorFilter). For now, SkRuntimeEffect::Make uses kRuntimeEffect,
which continues to be more lenient and allow creation of either shaders
or color filters from a single effect. After we migrate all clients, we
can deprecate and then delete that mode.

Bug: skia:11813
Change-Id: I0afd79a72beeec84da42c86146e8fcd8d0e4c09f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395716
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
24 files changed