dsound: Precache PerfectPitch(TM) and friends to lighten mixer load.

[Maarten Lankhorst]

By doing the resampling/transforming of the data outside the mixer
thread, it will only have to do ideal mixing, which is lighter on the timer.

For this to work properly, I had to replace the critical section a
buffer has by a read/write lock, so mixing from secondary buffer to
intermediary buffer and intermediate buffer to primary can be done at
the same time, which is what happened before the patch too, the rw lock
is to make sure the tmp_buffer isn't freed while mixing.
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