ffmbc 0.7 rc7 -tardet dvd ads a scale when it doesn't need to

If I craft an ffmbc command line with -target dvd to take IMX30 and crop the 
top 32 lines off, FFmbc adds an extra scape which scales the 608 lines into 576 
before the crop filter can do the crop.

In this example note how the first scale filter appears to have been inserted 
before the command line's scale filter (there just to change pixel format)

ffmbc -loglevel debug -i "6609305.mxf" -target dvd -vf 
scale=0:0:interl=1,format=yuv420p,crop=720:576:0:32 -y out.mpeg
FFmbc version 0.7-rc7
Copyright (c) 2008-2012 Baptiste Coudurier and the FFmpeg developers
[mxf @ 0x7f801a807600] Format mxf probed with size=2048 and score=100
[mxf @ 0x7f801a807600] All info found
[mxf @ 0x7f801a807600] 0: start_time: 0 duration: 2223
[mxf @ 0x7f801a807600] 1: start_time: 0 duration: 2223
Input #0, mxf, from 'jupiter_6609305.mxf':
  Metadata:
    operational_pattern: Op1a
    system_timecode: 00:00:00:00
    timecode: 20:49:25:08
  Duration: 00:01:28.92, start: 0.000000, bitrate: 42599 kb/s
    Stream #0.0(und), 1, 1/25: Video: mpeg2video (4:2:2), yuv422p, 720x608i tff [PAR 608:405 DAR 16:9], tbc 2/50, 30000 kb/s, tbr 25/1, tba 25/1, 25.00 fps
    Stream #0.1(und), 1, 1/25: Audio: pcm_s16le, 48000 Hz, 4 channels, s16, 3072 kb/s
[buffer @ 0x7f801a401ac0] w:720 h:608 pixfmt:yuv422p tb:1/1000000 sar:608/405 
sws_param:
[scale @ 0x7f801a403040] w:720 h:608 fmt:yuv422p -> w:720 h:576 fmt:yuv420p 
flags:0x4
[scale @ 0x7f801a403640] w:720 h:576 fmt:yuv420p -> w:720 h:576 fmt:yuv420p 
flags:0x4
[crop @ 0x7f801a403f00] w:720 h:576 -> w:720 h:576
[ac3 @ 0x7f801a80d200] channel_layout not specified
[ac3 @ 0x7f801a80d200] No channel layout specified. The encoder will guess the 
layout, but it might be incorrect.
Output #0, dvd, to 'out.mpeg':
  Metadata:
    encoder: FFmbc 0.7
    Stream #0.0(und), 0, 1/90000: Video: mpeg2video, yuv420p, 720x576p [PAR 64:45 DAR 16:9], tbc 1/25, 6000 kb/s, 25.00 fps
    Stream #0.1(und), 0, 1/90000: Audio: ac3, 48000 Hz, quad, s16, 448 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
Press [q] to stop, [?] for help
[mpeg2video @ 0x7f801a808200] rc buffer underflow
frame= 2223 fps= 90 q=5.3 Lsize=   71392kB time=00:01:28.88 
bitrate=6580.1kbits/s               
video:65214kB audio:4863kB global headers:0kB muxing overhead 1.875965%
Original issue reported on code.google.com by mark.him...@gmail.com on 10 Jun 2012 at 7:47
pocarist / ffmbc

ffmbc 0.7 rc7 -tardet dvd ads a scale when it doesn't need to #102
