Guard SkMatrix::get*Scale[s]() against negative nearly-zero values.

tests/MatrixTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMath.h"
 #include "SkMatrix.h"
 #include "SkMatrixUtils.h"
@@ skipping 195 matching lines @@
     REPORTER_ASSERT(reporter, -SK_Scalar1 == perspX.getMinScale());
     REPORTER_ASSERT(reporter, -SK_Scalar1 == perspX.getMaxScale());
     success = perspX.getMinMaxScales(scales);
     REPORTER_ASSERT(reporter, !success);
 
     // skbug.com/4718
     SkMatrix big;
     big.setAll(2.39394089e+36f, 8.85347779e+36f, 9.26526204e+36f,
                3.9159619e+36f, 1.44823453e+37f, 1.51559342e+37f,
                0.f, 0.f, 1.f);
-    REPORTER_ASSERT(reporter, -SK_Scalar1 == perspX.getMinScale());
-    REPORTER_ASSERT(reporter, -SK_Scalar1 == perspX.getMaxScale());
     success = big.getMinMaxScales(scales);
     REPORTER_ASSERT(reporter, !success);
 
+    // skbug.com/4718
+    SkMatrix givingNegativeNearlyZeros;
+    givingNegativeNearlyZeros.setAll(0.00436534f, 0.114138f, 0.37141f,
+                                     0.00358857f, 0.0936228f, -0.0174198f,
+                                     0.f, 0.f, 1.f);
+    success = givingNegativeNearlyZeros.getMinMaxScales(scales);
+    REPORTER_ASSERT(reporter, success && 0 == scales[0]);
+
     SkMatrix perspY;
     perspY.reset();
     perspY.setPerspY(-SK_Scalar1 / 500);
     REPORTER_ASSERT(reporter, -SK_Scalar1 == perspY.getMinScale());
     REPORTER_ASSERT(reporter, -SK_Scalar1 == perspY.getMaxScale());
     scales[0] = -5;
     scales[1] = -5;
     success = perspY.getMinMaxScales(scales);
     REPORTER_ASSERT(reporter, !success && -5 * SK_Scalar1 == scales[0] && -5 * SK_Scalar1  == scales[1]);
 
@@ skipping 746 matching lines @@
         
         mat.mapPoints((SkPoint*)&dst[0].fLeft, (SkPoint*)&src.fLeft, 2);
         dst[0].sort();
         mat.mapRect(&dst[1], src);
         mat.mapRectScaleTranslate(&dst[2], src);
 
         REPORTER_ASSERT(r, dst[0] == dst[1]);
         REPORTER_ASSERT(r, dst[0] == dst[2]);
     }
 }