#31 drv_wasapi: improve playback stability under heavy system load, support MMCSS "Pro Audio"

[Stephane Denis]

• Increase the default WASAPI shared buffer from 10 ms to 20 ms.
• Honor the configured buffer duration in classic shared mode.
• Register the render thread with MMCSS using the "Pro Audio" profile.
• Start the render thread before starting the audio client.
• Protect MikMod mixer access with the driver mutex.
• Silence only the unwritten tail when VC_WriteBytes returns a partial buffer.
• Preallocate format-conversion storage outside the real-time callback.
• Improve COM initialization and lifetime handling.
• Align IAudioClient3 periods to the supported fundamental period.
• Tested successfully with the MikMod player under heavy system load.

Patch in 'integration' branch
8d44ed497b6baf72847f626e5a6b6d585e680572

[Ozkan Sezer]

Cherry-picked the commit to master. Applied a few follow-up commits: Make sure all are OK.