winegstreamer: Implement MFT_MESSAGE_COMMAND_DRAIN for aac decoder.

CW-Bug-Id: #21804
CW-Bug-Id: #22299

dlls/winegstreamer/audio_decoder.c

@@ -489,7 +489,18 @@ static HRESULT WINAPI transform_ProcessEvent(IMFTransform *iface, DWORD id, IMFM
 
 static HRESULT WINAPI transform_ProcessMessage(IMFTransform *iface, MFT_MESSAGE_TYPE message, ULONG_PTR param)
 {
-    FIXME("iface %p, message %#x, param %p stub!\n", iface, message, (void *)param);
+    struct audio_decoder *decoder = impl_from_IMFTransform(iface);
+
+    TRACE("iface %p, message %#x, param %p.\n", iface, message, (void *)param);
+
+    if (!decoder->wg_transform)
+        return MF_E_TRANSFORM_TYPE_NOT_SET;
+
+    if (message == MFT_MESSAGE_COMMAND_DRAIN)
+        return wg_transform_drain(decoder->wg_transform);
+
+    FIXME("Ignoring message %#x.\n", message);
+
     return S_OK;
 }