Expand/finish animated texture (ANIMDEFS) functionality
In (G)ZDoom, there are a lot of useful features for animated textures, like variable frame durations, oscillation, random order, etc. (see https://zdoom.org/wiki/ANIMDEFS)
SRB2's ANIMDEFS are already based on ZDoom's ANIMDEFS, and in p_spec.c
you can actually see the ANIMDEFS parser checking for certain keywords (OSCILLATE, PIC, RAND), but they turn out to be unsupported in SRB2. It would be nice if SRB2 could properly support them...