Support large scaling

[Clément Bœsch]

./ffmpeg -f lavfi -i testsrc=s=1920x1080 -vf scale=30:30 -f null -
ffmpeg version N-58440-g841c0aa Copyright (c) 2000-2013 the FFmpeg developers
  built on Nov 25 2013 10:23:58 with gcc 4.8.2 (GCC)
  configuration: --enable-gpl --enable-libx264 --enable-libmp3lame --enable-x11grab --enable-libvorbis --samples=/home/ux/fate-samples --enable-libfreetype --enable-libvpx --cpu=native --cc='ccache cc'
  libavutil      52. 54.100 / 52. 54.100
  libavcodec     55. 44.100 / 55. 44.100
  libavformat    55. 21.101 / 55. 21.101
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 91.100 /  3. 91.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, lavfi, from 'testsrc=s=1920x1080':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24, 1920x1080 [SAR 1:1 DAR 16:9], 25 tbr, 25 tbn, 25 tbc
sws: filterSize 256 is too large, try less extreme scaling or increase MAX_FILTER_SIZE and recompile
sws: initFilter failed
[Parsed_scale_0 @ 0x1d018c0] Failed to configure output pad on Parsed_scale_0
Error opening filters!

[Michael Niedermayer]

Fixed in fba894615d694584057adb0ddb4d609486cad807