UI: Add option to disable audio ducking on windows

The audio subsystem of windows is by default configured to lower the
volume of other things while a communications device (mic) is currently
active.  This patch prevents that from being enabled with OBS.  If the
user needs audio ducking enabled again for whatever reason, there is now
an option to re-enable it in advanced settings.

Closes jp9000/obs-studio#884

UI/platform-windows.cpp

@@ -30,6 +30,11 @@ using namespace std;
 #include <shellapi.h>
 #include <shlobj.h>
 #include <Dwmapi.h>
+#include <mmdeviceapi.h>
+#include <audiopolicy.h>
+
+#include <util/windows/HRError.hpp>
+#include <util/windows/ComPtr.hpp>
 
 static inline bool check_path(const char* data, const char *path,
 		string &output)
@@ -216,3 +221,41 @@ void SetWin32DropStyle(QWidget *window)
 	ex_style |= WS_EX_ACCEPTFILES;
 	SetWindowLongPtr(hwnd, GWL_EXSTYLE, ex_style);
 }
+
+bool DisableAudioDucking(bool disable)
+{
+	ComPtr<IMMDeviceEnumerator>   devEmum;
+	ComPtr<IMMDevice>             device;
+	ComPtr<IAudioSessionManager2> sessionManager2;
+	ComPtr<IAudioSessionControl>  sessionControl;
+	ComPtr<IAudioSessionControl2> sessionControl2;
+
+	HRESULT result = CoCreateInstance(__uuidof(MMDeviceEnumerator),
+			nullptr, CLSCTX_INPROC_SERVER,
+			__uuidof(IMMDeviceEnumerator),
+			(void **)&devEmum);
+	if (FAILED(result))
+		return false;
+
+	result = devEmum->GetDefaultAudioEndpoint(eRender, eConsole, &device);
+	if (FAILED(result))
+		return false;
+
+	result = device->Activate(__uuidof(IAudioSessionManager2),
+			CLSCTX_INPROC_SERVER, nullptr,
+			(void **)&sessionManager2);
+	if (FAILED(result))
+		return false;
+
+	result = sessionManager2->GetAudioSessionControl(nullptr, 0,
+			&sessionControl);
+	if (FAILED(result))
+		return false;
+
+	result = sessionControl->QueryInterface(&sessionControl2);
+	if (FAILED(result))
+		return false;
+
+	result = sessionControl2->SetDuckingPreference(disable);
+	return SUCCEEDED(result);
+}