Clamp negative sigma to 0 in SkDropShadowImageFilter.

This can occur when the matrix includes a perspective transform.

BUG=skia:2094
R=bsalomon@google.com

Review URL: https://codereview.chromium.org/137833004

git-svn-id: http://skia.googlecode.com/svn/trunk@13220 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed