Fixed various issues with the ways certain user WADs defined custom skies for Doom 2 levels. New, simpler ZScript-based detection and substitution logic replaces the old ACS-based method.
Properly detect SIGIL under any of its released names, eg SIGIL_V1_0, SIGIL_V1_1, SIGIL_V1_2, SIGIL_V1_21. Creates a copy in source_wads/ with the expected name of "sigil.wad".
Fixed episode listing appearing in the smaller GZDoom built-in font instead of main Doom menu font. Thanks TwelveEyes.
When playing a map with a Doom 1 style ExMx name, swap in alternate versions of BRNPOIS and NUKEPOIS that are the correct sizes, in case any user made levels depend on that difference.
Changed the included graphics (eg menu title, episode names) from 32-bit to 8-bit so they work correctly with palette mods.