Re-add/fix broken platform momz mobj code.

The changes in this branch re-add the platform's momentum to players and mobjs which leave the platform (eg. by jumping) so that they move with relative velocity as expected. This behavior was unintentionally broken in SRB2 2.0, which adds a lot of artificial difficulty to certain segments of the levels, where you have to jump between high velocity moving platforms which seemingly cut your jump height to nothing.

Not only has the behavior been fixed, but it has now been enhanced to move the camera while free-falling between platforms as well, completing the illusion of full relative velocity with minimal hiccups. Observe.