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.

BUG=chromium:682918

Change-Id: I5a394098665d01e995a244fde278236f1471e6c9
Reviewed-on: https://skia-review.googlesource.com/8328
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
4 files changed