Make `resize` dependency optional

[RunDevelopment]

I think the resize dependency should be optional. Users that don't need encoding or don't need mipmap generation will never use it, so I think they should have the option to disable it.

The question becomes how to do when resize is not available. I can think of 3 options:

1. Runtime error: Just return an error when the user requests for mipmaps to be generated. The API will stay exactly the same.
2. Fallback: It's very easy to implement a fast NN interpolation, so we could ignore the requested resize filter and just always use NN. Alternatively, we could return an error when a filter that requires resize is used (so option 1+2).
3. #[cfg] everything: Make it so that all API related to mipmap generation are only available when resize is enabled. This has the huge downside that the API of the library depends on a feature.