expand SkCanvas with doDrawSlug, etc.

Follow the ordinary pattern on SkCanvas for handling GrSlugs. This
should allow more flexibility in the future.

Change-Id: Ic498b5d081a1c7e44c2864439423910c452408b7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/474899
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Herb Derby <herb@google.com>
4 files changed