Disable MVE mixer when movie ends

The global variables that control it configure the callback to do
nothing, but SDL_mixer would still call it and still acquire the SDL
mutex that protects the call.  Such calls are a waste, so disable them.
This should also fix
<https://github.com/dxx-rebirth/dxx-rebirth/issues/684>, which appears
to be caused by the MVE mixer callback being run during gameplay, and
the callback not returning immediately.  This is suspected to be because
a race condition between MVE_rmEndMovie and the callback caused the
global variables not to be placed in a state that causes the callback to
return immediately.

d2x-rebirth/libmve/mveplay.cpp

@@ -746,6 +746,10 @@ void MVE_rmEndMovie(std::unique_ptr<MVESTREAM>)
 		{
 			SDL_CloseAudio();
 		}
+#if DXX_USE_SDLMIXER
+		else
+			Mix_SetPostMix(nullptr, nullptr);
+#endif
 		mve_audio_canplay = 0;
 	}
 	mve_audio_buffers = {};