Add IsAnimated property (alternatively: FrameCount) to ImageInfo

[zotanmew]

This would make special handling of animated images (e.g., to skip processing them) trivial.

[zotanmew]

I wrote some code for this: https://iceshrimp.dev/iceshrimp/ImageSharp/commit/7ab7ff4b6a74acd4f32381d93d1e8e5159a3f6dc

Happy to submit a PR if wanted.

[JimBobSquarePants]

Hi @zotanmew Thanks for the suggestion but unfortunately the presence of multiple frames does not automatically mean that the image is animated. Container formats like Tiff for example can have multiple frames.

[zotanmew]

@JimBobSquarePants How about FrameCount? Would be similarly useful without that issue. I fully understand if you aren't interested in upstreaming this, however.

[JimBobSquarePants]

V4 will contain an explicit FrameCount property on ImageInfo as part of #2751