show flatness vectors in sample

Bug: skia:
Change-Id: I5a3882c993138dc5e1da92a7a4b1a9f96c740d1c
Reviewed-on: https://skia-review.googlesource.com/121461
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed