[SDL] Mix_LoadMUS_RW in SDL_mixer 1.2.4

[Ryan C. Gordon]

> You don't look busy; send a patch.  :)

Specifically, from SDL_mixer.h:

#if 0 /* This hasn't been hooked into music.c yet */
/* Load a music file from an SDL_RWop object (MikMod-specific currently)
   Matt Campbell (matt at campbellhome.dhs.org) April 2000 */
extern no_parse_DECLSPEC Mix_Music * SDLCALL Mix_LoadMUS_RW(SDL_RWops *rw);
#endif

In case you haven't seen that. It's probably a mixture of developer
inertia and the fact that adding an API call just for mikmod is a little
yucky.

--ryan.