libopenmpt library support

This merge requests adds support for libopenmpt, a cross-platform C++ and C library to decode tracked music files (modules). The difference compared to the internal library SDL_Mixer uses is that it's more accurate, supporting more formats. This also supports proper looping, and changing the tempo of the song(Note that the library only supports values of 0 to 4, as float. Anything higher will just silently error out and do nothing else). Since modifying SDL_Mixer directly is out of the question, this basically runs in place. When the loaded music is detected to be module based, control is automatically passed to the library.

A example wad is provided. tunes test1 to test4 in console.

libopenmpt-test.wad