Mark MS-MPEG4v1 encoder as experimental.

The encoder has never produced files that could be decoded
with any software.

libavcodec/msmpeg4.c

@@ -1917,7 +1917,7 @@ AVCodec ff_msmpeg4v1_decoder = {
     NULL,
     ff_h263_decode_end,
     ff_h263_decode_frame,
-    CODEC_CAP_DRAW_HORIZ_BAND | CODEC_CAP_DR1,
+    CODEC_CAP_DRAW_HORIZ_BAND | CODEC_CAP_DR1 | CODEC_CAP_EXPERIMENTAL,
     .max_lowres= 3,
     .long_name= NULL_IF_CONFIG_SMALL("MPEG-4 part 2 Microsoft variant version 1"),
     .pix_fmts= ff_pixfmt_list_420,