WebPImageCodec does not correctly decode animated WEBP images

[gsemac]

Since #6 was resolved, animated WEBP images no longer throw an exception when decoding. However, the decoded image is simply a black box. This appears to be a problem with how WebP-wrapper is implemented: JosePineiro/WebP-wrapper/issues/27

This could be resolved by implementing the decoder myself and forgoing the dependency on WebP-wrapper, or adding special handling for animated WEBP images, which can already be detected through the Animation property on IImage.