Mouse position for 2D scene

Wrapper for the mousePosition to be on the right place in a Scene::SCENE_2D as mentioned in #445 

Example: 
```cpp
void HelloPolycodeApp::handleEvent(Event *e) {
	
	if(e->getDispatcher() == core->getInput()) {
		InputEvent *inputEvent = (InputEvent*)e;
		switch(e->getEventCode()) {
			case InputEvent::EVENT_MOUSEMOVE:
				image->setPosition(core->getInput()->getMousePositionScene2D().x,
						core->getInput()->getMousePositionScene2D().y);
			break;
		}
	}
}
```