Add pinch-to-zoom simulation

If Ctrl is hold when the left-click button is pressed, enable
pinch-to-zoom to scale and rotate relative to the center of the screen.

Fixes #24 <https://github.com/Genymobile/scrcpy/issues/24>

app/src/input_manager.h

@@ -30,6 +30,8 @@ struct input_manager {
         unsigned data[SC_MAX_SHORTCUT_MODS];
         unsigned count;
     } sdl_shortcut_mods;
+
+    bool vfinger_down;
 };
 
 void