move top-level Slug creation from buffer to GrSlug

Change-Id: Ieef75feb64614a6ece4c88deaaf147fe1ac0b311
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/510420
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed