Lift the tessellation atlas into its own path renderer

Creates a new path renderer, GrAtlasPathRenderer, that handles all the
atlasing. Managing the atlas in its own path renderer gives us more
control over when atlasing happens in the chain, will allow us to more
easily use the atlas in kCoverage mode, and makes the clipping code
cleaner.

Bug: skia:12258
Change-Id: Ie0b669974936c23895c8ab794e2d97206ed140f8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/431896
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
23 files changed