Allow enabling wasapi exclusive initialisation via environment variable

`OSU_AUDIO_WASAPI_EXCLUSIVE` can now be set to `1` to test experimental
exclusive WASAPI support. Hopefully reduces latency and doesn't cause
adverse issues this time around.

Author: peppy

osu.Framework/Threading/AudioThread.cs

@@ -138,9 +138,11 @@ internal bool InitDevice(int deviceId)
             if (!Bass.Init(deviceId, Flags: (DeviceInitFlags)128)) // 128 == BASS_DEVICE_REINIT
                 return false;
 
-            // Need to do more testing. Users reporting buffer underruns even with a large (20ms) buffer.
-            // Also playback latency improvements are not present across all users.
-            // attemptWasapiInitialisation();
+            // That this has not been mass-tested since https://github.com/ppy/osu-framework/pull/6651 and probably needs to be.
+            // Currently envvar gated for users to test at their own discretion.
+            bool useWasapiInit = Environment.GetEnvironmentVariable("OSU_AUDIO_WASAPI_EXPERIMENTAL") == "1";
+            if (useWasapiInit)
+                attemptWasapiInitialisation();
 
             initialised_devices.Add(deviceId);
             return true;