I've fixed the issues with the DirectSound driver which a number of commits from 2016 attempted to address. Additionally, build testers strongly prefer the fixed DirectSound to WaveOut (the current default driver for Windows) in terms of both game stability and sync stability, so I'm making it the default driver as well.
StepMania commit 75a9532 from 2006 had a subtle error, the declaration of len and locked_buf will hide the previous local declaration of these variables. The issues causing the DirectSound driver to be unreliable in terms of sync were due to this. This PR renames the variables from the 2006 commit to prevent this, and also ensures all variables are properly initialized.
Renamed `locked_buf` and `len` in `MixerThread` to `locked_buf_init` and `len_init` to avoid shadowing.
Prevent use of uninitialized member variables by initializing `m_pPCM`, `m_iSampleRate`, and `m_bShutdownMixerThread` in the constructor.
