When outsetting for rect blur fast path use abs of scale factors.

Fixes an issue where negative scales caused an inset rather than an outset.

Add GM.

Bug: chromium:899512
Change-Id: I9164c76da479af80d4f5389b057ec52a946726fb
Reviewed-on: https://skia-review.googlesource.com/c/171641
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed