software audio mixer

A new custom software mixer, supports playing streams at any rate, which is needed to emulate the directsound SetFrequency effects (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. Support for FLAC and other streaming formats can be added.
Add SOUND_COUNT to the end of the RCT2_SOUND enum and use this instead of 63.