first collision concept

-  the camera now has a sphere collision model
- upon collision all movement is nullified, which then feels like the
camera is getting stuck against the wall.
- the most natural thing to do is to just add the collision normal so
that the camera slides along the wall, never trespassing it
final collision concept

- now the manipulator does not get stuck. it performs only the
necessary correction to ensure that the motion does not penetrate the
object.
Merge branch 'collidable-camera'