Fix up signed-integer-overflow warnings

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 "SkMatrix.h"
 #include "SkFloatBits.h"
 #include "SkRSXform.h"
@@ skipping 138 matching lines @@
         m01 = m01 != 0;
         m10 = m10 != 0;
 
         int dp0 = 0 == (m00 | m11) ;  // true if both are 0
         int ds1 = m01 & m10;        // true if both are 1
 
         mask |= (dp0 & ds1) << kRectStaysRect_Shift;
     } else {
         // Only test for scale explicitly if not affine, since affine sets the
         // scale bit.
-        if ((m00 - kScalar1Int) | (m11 - kScalar1Int)) {
+        if ((m00 ^ kScalar1Int) | (m11 ^ kScalar1Int)) {
             mask |= kScale_Mask;
         }
 
         // Not affine, therefore we already know secondary diagonal is
         // all zeros, so we just need to check that primary diagonal is
         // all non-zero.
 
         // map non-zero to 1
         m00 = m00 != 0;
         m11 = m11 != 0;
@@ skipping 1603 matching lines @@
     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
 }