specialize setConcat for scale+translate

include/core/SkMatrix.h

Index: include/core/SkMatrix.h
diff --git a/include/core/SkMatrix.h b/include/core/SkMatrix.h
index 9f44bed9c4da20251ccb2053d3b9e2ce7dcec2ee..b3c7378ab27dce136832b910fcaccab9ae924dcc 100644
--- a/include/core/SkMatrix.h
+++ b/include/core/SkMatrix.h
@@ -646,6 +646,29 @@ private:
     SkScalar         fMat[9];
     mutable SkTRacy<uint32_t> fTypeMask;
 
+    void setScaleTranslate(SkScalar sx, SkScalar sy, SkScalar tx, SkScalar ty) {
+        fMat[kMScaleX] = sx;
+        fMat[kMSkewX]  = 0;
+        fMat[kMTransX] = tx;
+        
+        fMat[kMSkewY]  = 0;
+        fMat[kMScaleY] = sy;
+        fMat[kMTransY] = ty;
+        
+        fMat[kMPersp0] = 0;
+        fMat[kMPersp1] = 0;
+        fMat[kMPersp2] = 1;
+        
+        unsigned mask = 0;
+        if (sx != 1 || sy != 1) {
+            mask |= kScale_Mask;
+        }
+        if (tx || ty) {
+            mask |= kTranslate_Mask;
+        }
+        this->setTypeMask(mask | kRectStaysRect_Mask);
+    }
+
     uint8_t computeTypeMask() const;
     uint8_t computePerspectiveTypeMask() const;