Port over unique Zone Builder features

SRB2-specific rendering:

• NiGHTS paths
• Zoom tube sequences
• Polyobject previews

Zone Builder Unofficial features:

• Parallel/perpendicular placement modes
• Place things at vertices
• and more

Actions:

• Select non-essential vertices/lines (Classic Modes)*
• Reset actions/flags/tags*
• Select floors/ceilings (Visual Mode)
• Deselect FOF planes (Visual Mode)
• (De)select things (Visual Mode)
• Toggle invisible FOFs (Visual Mode)

Miscellaneous:

• Increase minimum thing render size*
• Change filtering on image export (perhaps as an option*)
• Look into reverting some blockmap-related "optimizations"
• Allow selecting 1-/2-sided only in Visual Mode*

Pushed to UDB:

• Add Tab to the list of special controls
• Move selection N/S/E/W (Edit Selection Mode)
• Fix "Back" option never being checked when loading default stair builder preset
• Increment angle by 5 instead of 8 in when curving linedefs
• Allow rotation actions in Edit Selection mode

* Potentially worth pushing to UDB directly