Linked Portal Door Concept: Changing Gravity 2 (Source SDK)

This is a more player friendly implementation of gravity changes. The player aims at what appears to be the ceiling, only to find that it turns out to be a floor. It also serves as a valuable test of the limits of portal gun portals between linked_portal_doors. There is a clear "seam" where the linked_portal_door is, and it blocks the placement of portals that would be half-in, half-out of a linked_portal_door