webp : implement fancy filtering

[mvignali]

Summary of the bug:
How to reproduce:

./ffmpeg -i ./rgb_webp_q80.webp res3.png
ffmpeg version N-80681-g4cc896e Copyright (c) 2000-2016 the FFmpeg developers
  built with Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
  configuration: --prefix=../build --disable-yasm --disable-ffplay --disable-ffprobe --disable-ffserver
  libavutil      55. 26.100 / 55. 26.100
  libavcodec     57. 46.100 / 57. 46.100
  libavformat    57. 40.101 / 57. 40.101
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 46.102 /  6. 46.102
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  1.100 /  2.  1.100
Input #0, webp_pipe, from './rgb_webp_q80.webp':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: webp, yuv420p(tv, bt470bg/unknown/unknown), 12x8, 25 tbr, 25 tbn, 25 tbc
[image2 @ 0x7f8ef1013800] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
Output #0, image2, to 'res3.png':
  Metadata:
    encoder         : Lavf57.40.101
    Stream #0:0: Video: png, rgb24, 12x8, q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc57.46.100 png
Stream mapping:
  Stream #0:0 -> #0:0 (webp (native) -> png (native))
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A speed=  24x    
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.

[mvignali]

png output with dwebp

[mvignali]

After more tests :
if we output yuv frame from webp sample using ffmpeg or dwebp, the files are binary exact.

When converting webp file to png with ffmpeg, the result is very close to the output of dwebp using the nofancy option.
./dwebp rgb_webp_q80.webp -nofancy -o res.png

[Carl Eugen Hoyos]

Our software scaler does not try to copy WebP's behaviour.