audio: stereo downmix, volume boost options (#722)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #717, fixes #647

## Description of change
Adds an option to downmix surround sound (5.1, 7.1, etc) down to stereo
(2 speakers). This can be used in most WASAPI games, including Gitadora
and FTT.

How it works: when the game tries to open surround format (say, 7.1) we
create a fake buffer and tell the game that it is supported. In reality
we open a 2-channel stream with the real sound card. When the stream
begins, we downmix the channels down to two (using one of many
algorithms) and output to the sound card.

While we're here, implement an option to boost the game volume by some
decibel value, which is a feature often requested by SDVX players. This
was needed since downmixing can sometimes result in quieter audio.

## Testing
Tested GW Delta and FTT.

Downmix doesn't work on IIDX (32 bits) but volume boost works.

src/spice2x/hooks/audio/backends/wasapi/audio_client.h

@@ -8,6 +8,8 @@
 #include "hooks/audio/audio_private.h"
 #include "util/logging.h"
 
+#include "downmix.h"
+
 #include "audio_render_client.h"
 
 // {1FBC8530-AF3E-4128-B418-115DE72F76B6}
@@ -93,4 +95,12 @@ struct WrappedIAudioClient : IAudioClient3 {
     AudioBackend *const backend;
     bool exclusive_mode = false;
     int frame_size = 0;
+
+    // the format the real device was opened with (after any downmix). used to scale the final
+    // output buffer for the volume boost.
+    WAVEFORMATEXTENSIBLE device_format = {};
+
+    // surround -> stereo downmix. the real device is opened as stereo while the game keeps
+    // writing multi-channel audio into a scratch buffer that we downmix in the render client.
+    hooks::audio::Downmix downmix;
 };