split subrun API into Blob and Op interfaces

Instead of using sub classing to generate the atlas API, give
the atlas portion its own interface. Then the atlas subrun
variants can subclass both interfaces.

Change-Id: I8a0ca3d19bd362877224fa64f6c49a5f50d0ceb5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/336958
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed