Implement touch control

tools/viewer/Viewer.h

Index: tools/viewer/Viewer.h
diff --git a/tools/viewer/Viewer.h b/tools/viewer/Viewer.h
index 033bd6969b9684a7c8d10ca474583876e0456b49..13cfada9d205a08372ef101f2b51c5be80bf3460 100644
--- a/tools/viewer/Viewer.h
+++ b/tools/viewer/Viewer.h
@@ -24,6 +24,7 @@ public:
 
     void onPaint(SkCanvas* canvas);
     void onIdle(double ms) override;
+    bool onTouch(int owner, sk_app::Window::InputState state, float x, float y);
 
 private:
     void initSlides();
@@ -33,7 +34,7 @@ private:
     void drawStats(SkCanvas* canvas);
 
     void changeZoomLevel(float delta);
-    void updateMatrix();
+    SkMatrix computeMatrix();
 
     sk_app::Window*        fWindow;
 
@@ -48,13 +49,14 @@ private:
     bool                   fDisplayStats;
 
     // transform data
-    SkMatrix               fLocalMatrix;
     SkScalar               fZoomCenterX;
     SkScalar               fZoomCenterY;
     SkScalar               fZoomLevel;
     SkScalar               fZoomScale;
 
     sk_app::CommandSet     fCommands;
+
+    SkTouchGesture         fGesture;
 };