Prevent outset Ws from going negative

Bug: skia:9028
Change-Id: I8e3d37050d3fce7602eee62ae911eae756e603a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211100
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed