introduce the GrSlug API

Introduce the GrSlug that encapsulates an SkTextBlob along with
the origin, and paint to create a GrSlug. You can draw the slug
using the canvas, and it will obey the canvas's clip and mapping.

See the full working prototype at:
  https://skia-review.googlesource.com/c/skia/+/471157/17

Change-Id: I5731dce3dea316ad144baead154a68c8e4c5db9a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/474040
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
5 files changed