Skins refactor

I used my old ports chopping block axe to cut off the tail end of the fearsome r_things.c dragon, and it turned into a new file: r_skins.c! (likewise I made a corresponding header file named r_skins.h to go with it)

WIP for now since I may make some further cleanup before saying this is ready to be merged in (though technically this already could go in as it is, as it does at least compile) Eh, since it can compile, may as well not make it WIP and just allow it to be merged in.