Added Module function 'set_song_position(int position)'

[BitJag]

Changes the current playback position of the song to the start of the pattern at the given song position.

Useful for:

1. Changing the point in the song as a reaction to something happening in game.
2. Faster than having to stop and clear the currently loaded module, and then loading a new module.
3. Can potentially save space by not having to duplicate the same sample for an instrument that happens to be used in two different MOD files. For example: If you have two songs that use the same samples, you can compose both songs inside of the same MOD file, avoiding duplicate sample data if you were to instead use two separate MOD files.