Support decoding animated WebP images

[urvang]

Summary of the bug:
Currently, FFmpeg can encode animated WebPs?, but cannot decode them, which is inconvenient for users.

Decoding animated WebPs? right now generates "image data not found error" due to "unsupported chunk": ​https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/webp.c#L1504

How to reproduce:
(example input image: ​https://www.gstatic.com/webp/animated/1.webp)

% ffmpeg -i 1.webp 1.gif
ffmpeg version N-75787-g1d4af04 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
  configuration: --enable-libwebp
  libavutil      55.  2.100 / 55.  2.100
  libavcodec     57.  4.100 / 57.  4.100
  libavformat    57.  3.100 / 57.  3.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 10.100 /  6. 10.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
[webp @ 0x3782940] image data not found
[webp_pipe @ 0x3781360] decoding for stream 0 failed

< Rest of the output truncated >

[urvang]

An animated WebP image