vc1dec: add flush function for WMV9 and VC-1 decoders

CC: libav-stable@libav.org
(cherry picked from commit 4dc8c8386e)

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 02b7239462)

Conflicts:
	libavcodec/vc1dec.c

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 0173a7966b)

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit aa41212767)

Conflicts:
	libavcodec/vc1dec.c

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/vc1.c

@@ -4347,6 +4347,7 @@ AVCodec vc1_decoder = {
     vc1_decode_frame,
     CODEC_CAP_DELAY,
     NULL,
+    .flush          = ff_mpeg_flush,
     .long_name = NULL_IF_CONFIG_SMALL("SMPTE VC-1"),
     .pix_fmts = ff_pixfmt_list_420
 };
@@ -4362,6 +4363,7 @@ AVCodec wmv3_decoder = {
     vc1_decode_frame,
     CODEC_CAP_DELAY,
     NULL,
+    .flush          = ff_mpeg_flush,
     .long_name = NULL_IF_CONFIG_SMALL("Windows Media Video 9"),
     .pix_fmts = ff_pixfmt_list_420
 };