Allow defined functions in the SkSL .inc files

Use that to implement many intrinsics in SkSL for the interpreter.

Change-Id: I3762867781cb1a053429fd37b12ae3cf7739cb3c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230134
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed