avdevice/dshow: set no-seek flags

avdevice/dshow is a realtime device and as such does not support
seeking. Therefore, its demuxer format should define the
AVFMT_NOBINSEARCH, AVFMT_NOGENSEARCH and AVFMT_NO_BYTE_SEEK flags.
With these flags set, attempting to seek (with, e.g.,
avformat_seek_file()) correctly yields -1 (operation not permitted)
instead of -22 (invalid argument).

This actually seems to apply to many other devices, at least the
gdigrab, v4l2, vfwcap, x11grab, fbdev, kmsgrab and android_camera
devices, from reading the source.

Signed-off-by: Diederick Niehorster <dcnieho@gmail.com>
Reviewed-by: Roger Pack <rogerdpack2@gmail.com>

libavdevice/dshow.c

@@ -1329,6 +1329,6 @@ const AVInputFormat ff_dshow_demuxer = {
     .read_header    = dshow_read_header,
     .read_packet    = dshow_read_packet,
     .read_close     = dshow_read_close,
-    .flags          = AVFMT_NOFILE,
+    .flags          = AVFMT_NOFILE | AVFMT_NOBINSEARCH | AVFMT_NOGENSEARCH | AVFMT_NO_BYTE_SEEK,
     .priv_class     = &dshow_class,
 };