Changes:
Implement software surround downmixing (fix#796).
Fix a crash when no audio renderer were created when stopping emulation.
NOTE: This PR also disable support of 5.1 surround on the OpenAL backend as we cannot detect if the hardware directly support it. (the downmixing applied by OpenAL on Windows is terribly slow)
* Audio: Implement libsoundio as an alternative audio backend
libsoundio will be preferred over OpenAL if it is available on the machine. If neither are available, it will fallback to a dummy audio renderer that outputs no sound.
* Audio: Fix SoundIoRingBuffer documentation
* Audio: Unroll and optimize the audio write callback
Copying one sample at a time is slow, this unrolls the most common audio channel layouts and manually copies the bytes between source and destination. This is over 2x faster than calling CopyBlockUnaligned every sample.
* Audio: Optimize the write callback further
This dramatically reduces the audio buffer copy time. When the sample size is one of handled sample sizes the buffer copy operation is almost 10x faster than CopyBlockAligned.
This works by copying full samples at a time, rather than the individual bytes that make up the sample. This allows for 2x or 4x faster copy operations depending on sample size.
* Audio: Fix typo in Stereo write callback
* Audio: Fix Surround (5.1) audio write callback
* Audio: Update Documentation
* Audio: Use built-in Unsafe.SizeOf<T>()
Built-in `SizeOf<T>()` is 10x faster than our `TypeSize<T>` helper. This also helps reduce code surface area.
* Audio: Keep fixed buffer style consistent
* Audio: Address styling nits
* Audio: More style nits
* Audio: Add additional documentation
* Audio: Move libsoundio bindings internal
As per discussion, moving the libsoundio native bindings into Ryujinx.Audio
* Audio: Bump Target Framework back up to .NET Core 2.1
* Audio: Remove voice mixing optimizations.
Leaves Saturation optimizations in place.
