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.](http://i.imgur.com/zmSfUyp.gifv)
See merge request !14
This fixes the issue with upside-down springs shooting downwards if you touch another of its kind. Also fixes one of the issues with monitors in Icicle Falls (after you phase inside the East-most float-bob FOF's monitor via the other bug and jump up to break it while there, the NEXT monitor moves upwards too)
Here's how it works: When a player walks off the
moving platform, it applies their pmomz once, and
then _keeps pmomz set_ so that the camera still
adds pmomz to its movements until they hit another
floor. This way, the camera doesn't jerk around.
Also gain velocity from walking off an "up" elevator normally?
This _looks_ incorrect because the camera stops matching
the platform movement the moment you step off, but I
assure you it is a correct and accurate movement.
(Try it with chasecam off.)
Now players will apply platform movement when jumping,
but only if the platform is moving the same direction
as their jump is, and all other objects will have an
appropriate pmomz in reverse gravity FOF situations.
These functions were already here before, and I /swear/ the slope
physics became slightly less glitchy after switching to them...
Only issue is the slope plane mapping code hasn't been properly
converted yet, so they don't render properly for now.
