Some small improvements

Thanks for the awesome Duke3D code review (haven't gone through the "engine internals" chapter, yet). Reminded me to make a pull request...

My changes are quite small - it's what caught my eye while browsing through the code:

- various long -> int32_t replace glitches
- allow music to be loaded from outside the GRP file (it was always loaded from GRP)
- add latest SDL and SDL_mixer header and binaries for easier compilation on Windows (5MB)

Don't know if you like the last point, though.
And btw. I did not have palette issues on Win7 and VS2012.