Use SDF path miplevels based on the original path's size

Should produce sharper results than arbitrary fixed sizes.
Adds a new test to pathfill GM.

Was: https://skia-review.googlesource.com/c/8328/

BUG=chromium:682918, skia:6238

Change-Id: Ia62ea5ce6b4a5ac2b8b51d06d57dc951d6c340b8
Reviewed-on: https://skia-review.googlesource.com/8384
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
4 files changed