Fix the assert and clamping in SkMatrix::get*Scale[s]().

src/core/SkMatrix.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkFloatBits.h"
 #include "SkMatrix.h"
 #include "SkNx.h"
@@ skipping 1553 matching lines @@
         } else if (kMax_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
             results[0] = apluscdiv2 + x;
         } else {
             results[0] = apluscdiv2 - x;
             results[1] = apluscdiv2 + x;
         }
     }
     if (!SkScalarIsFinite(results[0])) {
         return false;
     }
-    if (results[0] < 0 && results[0] > -SK_ScalarNearlyZero) {
+    // Due to the floating point inaccuracy, there might be an error in a, b, c
+    // calculated by sdot, further deepened by subsequent arithmetic operations
+    // on them. Therefore, we allow and cap the nearly-zero negative values.
+    SkASSERT(results[0] >= -SK_ScalarNearlyZero);
+    if (results[0] < 0) {
         results[0] = 0;
     }
-    SkASSERT(results[0] >= 0);
     results[0] = SkScalarSqrt(results[0]);
     if (kBoth_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
         if (!SkScalarIsFinite(results[1])) {
             return false;
         }
-        if (results[1] < 0 && results[1] > -SK_ScalarNearlyZero) {
+        SkASSERT(results[1] >= -SK_ScalarNearlyZero);
+        if (results[1] < 0) {
             results[1] = 0;
         }
-        SkASSERT(results[1] >= 0);
         results[1] = SkScalarSqrt(results[1]);
     }
     return true;
 }
 
 SkScalar SkMatrix::getMinScale() const {
     SkScalar factor;
     if (get_scale_factor<kMin_MinMaxOrBoth>(this->getType(), fMat, &factor)) {
         return factor;
     } else {
@@ skipping 286 matching lines @@
     const SkScalar m10 = -m01;
     const SkScalar m11 = m00;
     const SkScalar m12 = fTy;
 
     quad[0].set(m02, m12);
     quad[1].set(m00 * width + m02, m10 * width + m12);
     quad[2].set(m00 * width + m01 * height + m02, m10 * width + m11 * height + m12);
     quad[3].set(m01 * height + m02, m11 * height + m12);
 #endif
 }