WebPAnimEncoder API is a combination of encoder (WebPEncoder) and muxer
(WebPMux). It performs several optimizations to make it more efficient
than the combination of WebPEncode() and native ffmpeg muxer.

When WebPAnimEncoder API is used:
- In the encoder layer: we use WebPAnimEncoderAdd() instead of
  WebPEncode().
- The muxer layer: works like a raw muxer.

On the other hand, when WebPAnimEncoder API isn't available, the old code is
used as it is:
- In the codec layer: WebPEncode is used to encode each frame
- In the muxer layer:  ffmpeg muxer is used

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>