Q3BSP feature: portal camera effects

Quake 3 supports portal camera effects. Map estatica uses them to create fancy teleporters (vertical and horizontal), and ct3dm4 has a mirror effect.

The camera effects are enabled by map entities with classnames misc_portal_surface and misc_portal_camera.
DP is capable of effects like these and it would be great to have them working in Xon.

estatica in ioquake3 (CPMA)	estatica in Xon	estatica cam targets