Sector portals

Adds linedef type 6 for making one-way portals on sector planes, just like ZDoom's Sector_SetPortal

Parameters:

• tag: Tag of sectors in which the portal must be seen. Note that the sectors seen through the portal must not share this tag. For the 'copy to line' type this will be a line id, not a sector tag.
• type: The type of portal.
• plane: Whether the portal is set on the floor (0), the ceiling (1), or both planes (2).
• misc: The meaning of this argument depends on the type.
  • For a normal view portal (type 0): Specifies whether the line belongs to the sector viewed through the portal (1) or the sector in which the portal is seen (0).
  • For a transferred portal (type 1): Specifies the sector tag of the portal to copy.
  • For a sector-to-sector portal (type 7): Specifies the sector tag to make a portal to.
  • For a portal linked to an object (type 8): Specifies the object tag to make a portal to.

Types:

• 0: Normal view portal: Two linedefs must be defined, one in the "source" sector and one in the "destination" sector; the first three arguments for both of these linedefs must be identical while the fourth must be different. Any number of sectors can share this link by having the same tag. To make a two-way portal, another pair of linedef must be used.
  
• 1: Copied portal: 'misc' specifies the tag of the portal to copy. This copies the given portal to all sectors tagged with 'tag' or the line's front sector if 'tag' is 0. Use this for sectors which need to have a different tag, such as a lift, yet should share the portal.
• 2: Skybox portal: The linedef's front sector becomes a portal to the current skybox.
  
• 3: Plane portal (from Eternity Engine): Renders the linedef's front sector's planes into infinity at a fixed distance from the camera.
  
• 4: Horizon portal (from Eternity Engine): Renders the linedef's front sector's planes into infinity at the planes' heights (like linedef type 41.)
  
• 5: Copy portal to line: 'misc' specifies the portal to copy. If 'tag' is non-zero, copy the portal to all lines with the given ID, otherwise affects the line itself.
  
• 6: Do not use this.

Additionally, two new types are available:

• 7: Link to sector with second tag: Makes a portal to the tagged sector.
  
• 8: Link to object with second tag: Uses the tagged mapthing's spawned object as a viewpoint.