Updates for Continuous Zooming
Updates for Continuous Zooming

1. MapTile is a shared_ptr now. Ownership can be shared between visible tile set or proxy tile set
2. Added a loop to draw proxy tiles
3. Updated handlePinchGesture to call View::zoom appropriately.

quitejonny/tangram-es,hjanetzek/tangram-es,cleeus/tangram-es,karimnaaji/tangram-es,cleeus/tangram-es,cleeus/tangram-es,xvilan/tangram-es,hjanetzek/tangram-es,tangrams/tangram-es,hjanetzek/tangram-es,quitejonny/tangram-es,cleeus/tangram-es,quitejonny/tangram-es,tangrams/tangram-es,xvilan/tangram-es,tangrams/tangram-es,quitejonny/tangram-es,hjanetzek/tangram-es,xvilan/tangram-es,quitejonny/tangram-es,karimnaaji/tangram-es,tangrams/tangram-es,quitejonny/tangram-es,karimnaaji/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,karimnaaji/tangram-es,cleeus/tangram-es,xvilan/tangram-es,cleeus/tangram-es