Improve animated webp detection

[EricBAndrews]

Checklist

• I have read CONTRIBUTING.md
• I have described what this PR contains
• If this PR alters the UI, I have attached pictures/videos
• This PR addresses one or more open issues that were assigned to me:
  - Hopefully deals with Static webp sometimes laggy #1509

Pull Request Information

This PR improves how webp images are handled. Previously, every webp would go through the NukeWebpBridgeDecoder, which would both decode the image and, if the image was animated, populate the animated field. This had multiple problems:

• The BridgeDecoder was scanning the entire data to find the ANMF header
• The BridgeDecoder was therefore very slow, even for static images, and because the check was so slow it couldn't defer to the standard Nuke decoder for static images
• The BridgeDecoder was poorly optimized for static images, since it was built to handle animated images
• The BridgeDecoder could throw a false positive if ANMF happened to appear in the body of the image's data

This PR implements a much more efficient method of checking whether a webp is animated; it now scans only the critical bytes, rather than the whole data. This allows the NukeWebpBridgeDecoder to check at init whether a webp is animated and defer to the more efficient Nuke default decoder for static images.

Hopefully using Nuke for the static webps fixes the lagginess, which I believe may have been caused by the custom decoding. It may just be good luck, but I haven't encountered lag on this branch.

[Sjmarf]

LGTM! 💯