Warn that filtering is disabled when copying streams

[llogan]

Some users assume they can copy a video steam and perform filtering [1]. FFmpeg does not give any feedback that the filter is being ignored.

Current behavior:

$ ffmpeg -i hop.mp4 -c copy -vf crop=iw:ih-20 -y out.mpg
ffmpeg version N-35078-g7831788, Copyright (c) 2000-2011 the FFmpeg developers
  built on Nov 22 2011 10:36:32 with gcc 4.6.2
  configuration: --prefix=/usr --enable-gpl
  libavutil    51. 26. 0 / 51. 26. 0
  libavcodec   53. 36. 0 / 53. 36. 0
  libavformat  53. 21. 0 / 53. 21. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 49. 0 /  2. 49. 0
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'hop.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    creation_time   : 1970-01-01 00:00:00
    encoder         : Lavf52.32.0
  Duration: 00:00:10.13, start: 0.000000, bitrate: 563 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x480, 559 kb/s, 30 fps, 30 tbr, 30 tbn, 60 tbc
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : 
[mpeg @ 0x1d17860] VBV buffer size not set, muxing may fail
Output #0, mpeg, to 'out.mpg':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    creation_time   : 1970-01-01 00:00:00
    encoder         : Lavf53.21.0
    Stream #0:0(und): Video: h264 (avc1 / 0x31637661), yuv420p, 640x480, q=2-31, 559 kb/s, 30 fps, 90k tbn, 30 tbc
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : 
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
frame=  304 fps=  0 q=-1.0 Lsize=     700kB time=00:00:10.06 bitrate= 569.6kbits/s    
video:692kB audio:0kB global headers:0kB muxing overhead 1.166920%

Example warning: "Filtering is disabled when copying streams."

Of course the message could be more specific as to what streams are affected and what filters are actually disabled.

[1] http://ffmpeg.org/pipermail/ffmpeg-user/2011-November/003175.html