move perspective-clipper into SkPathPriv

From here we can start trying it out for real
- skottie could call it directly
- SkDraw could call it for the raster-backend

Change-Id: I703838241d67c539afd3d3e31d0ddf1c66645799
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/258001
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed