add optional "filter" type to image resize function

[aaron404]

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• I have added a new filter parameter to the resize_image function. It defaults to lanczos3 which is what was previously hardcoded. I had to add a mirror of the Filter type from the image crate to impl Hash for it. I'm not sure if there's a cleaner way to do this.
• I added a sprite from pixelfrog that has public domain usage rights to help demonstrate the differences of the filters in the updated docs.

[aaron404]

I'm not sure if I have incorrectly configured something, but it seems that running cargo test from the repo root does not execute all the tests that are used by the automated builder. I have a few test cases to fix up

[Keats]

cargo test will only run the root crate, you need to run cargo test --all to run tests for all the crates in a workspace

[Keats]

Can we convert directly to the filter type from the image crate and skip our own?

[aaron404]

I went with a custom type because FilterType from the image crate is not Hash, so it breaks downstream functionality when the processed image paths are generated.

[Keats]

I've just made a PR to impl it, let's see if it works

[Keats]

i think it's released in the most recent version