RoomCamera: allow follow avatar optionally

[mtwzim]

Just like the rooms of the habbos, the camera moves so that the avatar is in the center of the screen, it is currently possible to just place the room in the center of the screen.

Considering the following challenges (@jankuss):

1. Simple api:

   const avatar = new Avatar();
   const camera = new RoomCamera();
   camera.avatar = avatar;
   

2. Should be optional. RoomCamera should also work with no avatar provided