Use absolute sigma instead of clamping to 0 in SkDropShadowImageFilter.

If the CTM is scaled to negative value (without skew), mapVector returns a negative value. We want to use its absolute value rather than clamping to 0.

https://bugs.chromium.org/p/skia/issues/detail?id=9704

Change-Id: I7393a52aa7d646b1494d4f0ae1a520ab60d02104
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/259137
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

Author: paraboul

src/effects/imagefilters/SkDropShadowImageFilter.cpp

@@ -125,8 +125,8 @@ sk_sp<SkSpecialImage> SkDropShadowImageFilterImpl::onFilterImage(const Context&
 
     SkVector sigma = SkVector::Make(fSigmaX, fSigmaY);
     ctx.ctm().mapVectors(&sigma, 1);
-    sigma.fX = SkMaxScalar(0, sigma.fX);
-    sigma.fY = SkMaxScalar(0, sigma.fY);
+    sigma.fX = SkScalarAbs(sigma.fX);
+    sigma.fY = SkScalarAbs(sigma.fY);
 
     SkPaint paint;
     paint.setAntiAlias(true);