Camera should be able to slide along the world boundary

[st-pasha]

What could be improved

As discussed in #1902, whenever the target moves outside of camera bounds, the camera would "stick" to the boundary and remain there until the target moves back into the allowed region. A more natural behavior would be for the camera to "slide" along the camera boundary, so that it always remains as close as possible to the target within the allowed region.

Why should this be improved

Currently it is possible for the target to leave the camera boundary, and then travel along the boundary sufficiently far away that the camera would stop seeing the target -- even if the camera can be placed closer to the target to be able to see it, while still remaining within the allowed region.

Any risks?

The implementation needs to be smart enough to allow world boundaries of arbitrary shape.