Add perspective support to SkMatrix44 initializers.

include/utils/SkMatrix44.h

Index: include/utils/SkMatrix44.h
diff --git a/include/utils/SkMatrix44.h b/include/utils/SkMatrix44.h
index 6e85bb56f4bc2841852c30d4def5841a0ca26bd2..9282770588773d9628ca7d433376b405e6bac354 100644
--- a/include/utils/SkMatrix44.h
+++ b/include/utils/SkMatrix44.h
@@ -137,6 +137,14 @@ public:
         return !(other == *this);
     }
 
+    /* When converting from SkMatrix44 to SkMatrix, the third row and
+     * column is dropped.  When converting from SkMatrix to SkMatrix44
+     * the third row and column remain as identity:
+     * [ a b c ]      [ a b 0 c ]
+     * [ d e f ]  ->  [ d e 0 f ]
+     * [ g h i ]      [ 0 0 1 0 ]
+     *                [ g h 0 i ]
+     */
     SkMatrix44(const SkMatrix&);
     SkMatrix44& operator=(const SkMatrix& src);
     operator SkMatrix() const;
@@ -259,6 +267,8 @@ public:
     void setRowMajor(const SkMScalar data[]) { this->setRowMajord(data); }
 #endif
 
+    /* This sets the top-left of the matrix and clears the translation and
+     * perspective components (with [3][3] set to 1). */
     void set3x3(SkMScalar m00, SkMScalar m01, SkMScalar m02,
                 SkMScalar m10, SkMScalar m11, SkMScalar m12,
                 SkMScalar m20, SkMScalar m21, SkMScalar m22);