software audio mixer

A new custom software mixer, supports playing streams at any rate, which is needed to emulate the directsound SetFrequency effects (vehicle sounds play at different pitches and when a ride malfunctions and the music slows down and stops). The speex resampler is added for this. Currently samples can be played and their volumes and looping set on an arbitrary number of channels, and panning. Support for FLAC and other streaming formats can be added.  Replacement mixer sounds are not played at the moment and is meant to replace directsound once all sound related functions are figured out.  This will allow limits of the original audio to be extended, for example currently only 2 music tracks can be played at once and 7 channels of vehicle effects.