Apply sharp filters before upsizing

[abewieland]

Comparing results of this library with the ANSI C implementation by John Costella (at https://johncostella.com/magic/) led to differences when scaling up - the issue turned out to be that this library was applying the sharp filters after upscaling, which are supposed to be done before:

That implies that we must first apply the three-tap Sharp 2013 kernel, to the input image, before using the Magic Kernel to upsize the result.

(From here.)

I don't work in Rust often, so if there's a more idiomatic way to implement this, please feel free to close the PR and do so.