Door_Open(tag, speed, lighttag)
Opens a door, keeping it open.
- tag: tag of sectors to open. 0 means the sector behind the trigger line.
- speed: movement speed. Doom normal doors have 16 speed and turbo doors have 64.
- lighttag: optional tag for sector to get lit or darkened as door moves.
The door won't make any sound if it's already open.
The upper destination of a door sector ceiling is the lowest neighbour ceiling - 4. If a tagged sector's ceiling is higher that point, it will be moved instantly to that position.
For doors which close after a delay, see Door_Raise.