TRLE colours mode

[makotocchi]

The program needs a way of temporarily disabling the original textures and instead texture the level using TRLE colours (blue floor, green walls, pink triggers etc). This will make it much easier to discern squares and triggers.

• There should be an option in the view menu to toggle TRLE colours
• When selected, texture rendering should be disabled and replaced with TRLE colours

Notes:
Hidden geometry should be rendered, as it would be seen if it was the TRLE editor.
Walls are not currently rendered, so they'll have to be added when they're added.

• Add checkbox to view options to toggle TRLE colours mode
• Override colours for level mesh somehow
• Add tests
• Fix walls on portals
• Fix ceilings on walls
• Fix big columns
• Triangles are in the wrong direction on the floor (eg - SW -> NE instead of SE -> NW)
• Remove duplicate triangles
• Add grid instead of chequerboard
• Ceiling triangulation in TR3+
• Floor triangulation in TR3+ (diagonal half tiles)
• Use TRLE mesh for collision mesh