Revise SVD code to remove arctangents.

src/core/SkMatrixUtils.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkMatrixUtils_DEFINED
 #define SkMatrixUtils_DEFINED
 
@@ skipping 22 matching lines @@
 
 /**
  *  Calls SkTreatAsSprite() with default subpixelBits value to match Skia's
  *  filter-bitmap implementation (i.e. kSkSubPixelBitsForBilerp).
  */
 static inline bool SkTreatAsSpriteFilter(const SkMatrix& matrix,
                                          int width, int height) {
     return SkTreatAsSprite(matrix, width, height, kSkSubPixelBitsForBilerp);
 }
 
-/** Decomposes the upper-left 2x2 of the matrix into a rotation, followed by a non-uniform scale,
-    followed by another rotation. Returns true if successful.
-    If the scale factors are uniform, then rotation1 will be 0.
-    If there is a reflection, yScale will be negative.
-    Returns false if the matrix is degenerate.
+/** Decomposes the upper-left 2x2 of the matrix into a rotation (represented by
+    the cosine and sine of the rotation angle), followed by a non-uniform scale,
+    followed by another rotation. If there is a reflection, one of the scale 
+    factors will be negative.
+    Returns true if successful. Returns false if the matrix is degenerate.
     */
 bool SkDecomposeUpper2x2(const SkMatrix& matrix,
-                         SkScalar* rotation0,
-                         SkScalar* xScale, SkScalar* yScale,
-                         SkScalar* rotation1);
+                         SkPoint* rotation1,
+                         SkPoint* scale,
+                         SkPoint* rotation2);
 
 #endif