Issues with floor/ceiling conveyors

Making this issue for organization purposes.

• Momentum is handled incorrectly across sector edges, especially on sloped conveyor belts (#275 (closed), #481 (closed))

• Conveyor belts on FOF bottoms in reverse gravity require special sector properties (MSF_FLIPSPECIAL_CEILING or MSF_FLIPSPECIAL_BOTH), even though the conveyor belt sector type is no longer required in next. The game should figure this out by itself and add the respective sector flags automatically.

Test map: test_conveyors.wad