Try to fix test build on Windows.
Turn off LTCG status (it heavily spams the Jenkins logs).
Add mixxx-test alias to the mixxx-test SCons Program definition.
Check latency value more closely
Fix stuttering and loud buzzes when changing sound configuration.

Depending on your audio configuration, changing devices can cause stuttering
or (at low latencies) a loud buzz due to buffer under-runs caused by clearing
the clock reference device before all SoundDevices are closed. This causes the
engine to not process during callbacks where only some of the devices are closed
but the clock reference device is not closed yet.

* Don't clear m_pClkRefDevice in SoundManager::setupDevices(). This leads to
  dozens of buffer-underruns because the engine is not processed while we are
  closing every SoundDevice (which can block for a while).

* Some misc. SoundManager cleanups.
Add a comment describing the way forward for getting rid of needless re-opening
of devices when making audio hardware configuration changes.
Refactor the engine to process samples in the range of [-1.0, 1.0]. Fixes Bug #1204039.

* Move Vinyl Control pre-amp to VinylControlXwax (no longer affects
  microphone/passthrough).

* Switch PortAudio input sample format to paFloat32.

* Remove SHRT_MAX conversions on input and output samples.

* Update VUMeters, clipping, and dithering code to deal with the new ranges.

* Clean up EngineMicrophone/EnginePassthrough/EngineDeck handling of inputs.

* Update tests to deal with the new sample ranges.

This does not change SoundSource. SoundSources still read 16-bit samples and we
simply divide by SHRT_MAX in AnalyserQueue and CachingReaderWorker. A future
performance improvement can remove this and use the native decoder API to
request normalized samples directly.
Add protection against vinyl control pre-amp gains < 1.
Fix encoders to deal with new [-1.0, 1.0] internal normalization.
Fix comparison warning.
Dynamically allocate VinylControlXwax::m_pWorkBuffer.