winegstreamer: Set offset on GstBuffer.

Signed-off-by: Andrew Eikum <aeikum@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/winegstreamer/gstdemux.c

@@ -712,6 +712,8 @@ static GstFlowReturn request_buffer_src(GstPad *pad, GstObject *parent, guint64
         ERR("Returned %08x\n", hr);
         return GST_FLOW_ERROR;
     }
+
+    GST_BUFFER_OFFSET(*buf) = ofs;
     return GST_FLOW_OK;
 }