SkPictureShader should handle negative scaling gracefully.

src/core/SkPictureShader.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkPictureShader.h"
 
 #include "SkBitmap.h"
@@ skipping 139 matching lines @@
         m.preConcat(*localM);
     }
 
     // Use a rotation-invariant scale
     SkPoint scale;
     if (!SkDecomposeUpper2x2(m, NULL, &scale, NULL)) {
         // Decomposition failed, use an approximation.
         scale.set(SkScalarSqrt(m.getScaleX() * m.getScaleX() + m.getSkewX() * m.getSkewX()),
                   SkScalarSqrt(m.getScaleY() * m.getScaleY() + m.getSkewY() * m.getSkewY()));
     }
-    SkSize scaledSize = SkSize::Make(scale.x() * fTile.width(), scale.y() * fTile.height());
+    SkSize scaledSize = SkSize::Make(SkScalarAbs(scale.x() * fTile.width()),
+                                     SkScalarAbs(scale.y() * fTile.height()));
 
     // Clamp the tile size to about 16M pixels
     static const SkScalar kMaxTileArea = 4096 * 4096;
     SkScalar tileArea = SkScalarMul(scaledSize.width(), scaledSize.height());
     if (tileArea > kMaxTileArea) {
         SkScalar clampScale = SkScalarSqrt(SkScalarDiv(kMaxTileArea, tileArea));
         scaledSize.set(SkScalarMul(scaledSize.width(), clampScale),
                        SkScalarMul(scaledSize.height(), clampScale));
     }
 
@@ skipping 129 matching lines @@
     return bitmapShader->asFragmentProcessor(context, paint, viewM, NULL, paintColor, fp);
 }
 #else
 bool SkPictureShader::asFragmentProcessor(GrContext*, const SkPaint&, const SkMatrix&,
                                           const SkMatrix*, GrColor*,
                                           GrFragmentProcessor**) const {
     SkDEBUGFAIL("Should not call in GPU-less build");
     return false;
 }
 #endif