Clean up ALC sound device correctly.

ALC is thread-safe, therefore we can clean up from any thread and do not need to marshal to the main game thread. To clean up the device properly, we must first unset and destroy the context before attempting to close the device.

OpenRA.Platforms.Default/OpenAlSoundEngine.cs

@@ -49,6 +49,7 @@ namespace OpenRA.Platforms.Default
 		readonly Dictionary<uint, PoolSlot> sourcePool = new Dictionary<uint, PoolSlot>();
 		float volume = 1f;
 		IntPtr device;
+		IntPtr context;
 
 		static string[] QueryDevices(string label, int type)
 		{
@@ -103,10 +104,10 @@ namespace OpenRA.Platforms.Default
 					throw new InvalidOperationException("Can't create OpenAL device");
 			}
 
-			var ctx = ALC10.alcCreateContext(device, null);
-			if (ctx == IntPtr.Zero)
+			context = ALC10.alcCreateContext(device, null);
+			if (context == IntPtr.Zero)
 				throw new InvalidOperationException("Can't create OpenAL context");
-			ALC10.alcMakeContextCurrent(ctx);
+			ALC10.alcMakeContextCurrent(context);
 
 			for (var i = 0; i < PoolSize; i++)
 			{
@@ -303,17 +304,24 @@ namespace OpenRA.Platforms.Default
 
 		~OpenAlSoundEngine()
 		{
-			Game.RunAfterTick(() => Dispose(false));
+			Dispose(false);
 		}
 
 		public void Dispose()
 		{
-			Game.RunAfterTick(() => Dispose(true));
+			Dispose(true);
 			GC.SuppressFinalize(this);
 		}
 
 		void Dispose(bool disposing)
 		{
+			if (context != IntPtr.Zero)
+			{
+				ALC10.alcMakeContextCurrent(IntPtr.Zero);
+				ALC10.alcDestroyContext(context);
+				context = IntPtr.Zero;
+			}
+
 			if (device != IntPtr.Zero)
 			{
 				ALC10.alcCloseDevice(device);