Clean up in, about, and around GrTextBlob::SubRun

- Simplify ctors
- Remove using for VertexRegenerator - helps navigation in IDE
- Move SubRun and Run appendGlyph into paint to co-locate them

etc.

Change-Id: I6ef75a18e41cff577b8d4e9bb41ab8e312da532a
Reviewed-on: https://skia-review.googlesource.com/c/170540
Auto-Submit: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
5 files changed