Add support for Legacy-introduced linedef type 282: set sector colormap

[ioan-chera]

Some Boom-compatible wads like Hell Ground use type 282 for a different purpose than Eternity's: attaching 3DMidTex. Instead it's for applying a colourmap on tagged sectors. The feature comes from DOOM Legacy and checks the textures for RGB encoded names, and applies them according to rules. See the wiki article here: https://doomwiki.org/wiki/Linedef_type#Table_of_all_types and expand the table, then search for 282.

This can be implemented in Eternity if there's no 3DMidTex flag on any of the tagged lines, and if there are any RGB encoded names for the front side's textures. The big challenge would be about creating colormaps, because if we just make new ones in memory, it can become really wasteful if the level uses this linedef type everywhere with different colours. Maybe we should first allow computing light fading during drawing the same way we do with ZDoom-like translucency…