Added prototypes for DSLFunction

Previously, there was no way to create a forward declaration for a DSL
function. To avoid introducing new API and make this work in an
intuitive fashion, we now create prototypes for all DSL functions and
remove them when the function is promptly defined.

Change-Id: Ief36164ceb303a3d76a57dc073f2e9b8409bb45f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436562
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed