[BUG] Doom Retro can't parse duplicate flat marker lumps.

[MataKrow]

I have put 2 flat marker regions (FF_START/FF_END) in my wad.
Doom retro stops looking for flat markers when it reaches the first one.

examplewad.zip

[bradharding]

DOOM Retro merges flats (and sprites) using some old Chocolate DOOM code (w_merge.c, which is only designed to check for FF_START/FF_END once. I'll work on using BOOM's approach instead.