audio: WASAPI exclusive resampling, buffer size increase options (#727)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change

Resampler:
Implement resampler for exclusive mode streams, as we are seeing more
and more devices - not just laptops but onboard audio devices - that
only support 48khz and not 44.1khz. Should work with volume boost (gain
calculated inside resample) and also downmixer (hands off intermediate
scratch buffers).

Buffer size increase:
By default many of these games request a tiny buffer when in shared mode
(TDJ uses 3ms). On some audio setup this results in crackling due to
underflow. Add an option to forcibly increase the buffer size.

## Testing
With resampler set to 48kHz and buffer set to 20ms I can reliably boot
and play IIDX on my display port monitor's speakers; previously this
wasn't possible. IIDX is event-driven.

Tested SDVX7 as well at 48kHz, which opens timer-driven streams.

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

@@ -100,6 +100,14 @@ namespace hooks::audio {
         // grab the real stereo device buffer and hand the game the scratch buffer to write into.
         HRESULT get_buffer(IAudioRenderClient *real, UINT32 frames, BYTE **ppData);
 
+        // size the scratch and hand it to the game without acquiring a device buffer. used when a
+        // later stage (the resampler) owns the device interaction.
+        HRESULT get_scratch(UINT32 frames, BYTE **ppData);
+
+        // downmix the scratch the game wrote into the caller's stereo buffer, without touching the
+        // device. used to feed the resampler when the two stages are chained.
+        void downmix_into(BYTE *dst, UINT32 frames) const;
+
         // mix the scratch buffer into the stereo device buffer held since get_buffer. the caller
         // owns releasing the device buffer afterwards (see current_buffer / buffer_released).
         void write_device_buffer(UINT32 frames, DWORD flags);