mf: fix operation with --cache

demux_mf.c explicitly checks for the stream type to check whether images
are opened via pattern (mf://..., i.e. stream_mf.c) or directly. Of
course the stream type is not set to STREAMTYPE_MF if the stream is
wrapped through the cache, so it tried to open the pattern directly as
file, which failed.

Fix this by disabling caching for mf://. The cache doesn't make sense
here anyway, because each file is opened and closed every frame (perhaps
to avoid memory bloat).

stream/stream_mf.c

@@ -33,6 +33,7 @@ mf_stream_open (stream_t *stream, int mode)
 {
   stream->type = STREAMTYPE_MF;
   stream->demuxer = "mf";
+  stream->allow_caching = false;
 
   return STREAM_OK;
 }