2pt conical stage for concentric case

If the circles are concentric, we can use the radial stage and
post-massage t with a 2x3 stage.

Change-Id: I744bc889d509d5f9ff162c179e8ec7ca35410e3c
Reviewed-on: https://skia-review.googlesource.com/20972
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed