Integrate improvements from `resvg` crate

[Shnatsel]

resvg crate now includes a modified copy of the blur algorithm in this crate. Most notably it includes alpha channel support as well RGB/BRG/etc, but also fixes #13.

[fschutt]

Yeah, ideally this crate should be generic across array sizes, which is currently not stable in Rust.

[rljacobson]

Const generics are now in stable, so maybe it's time for another look.

Also useful would be an algorithm that operates on a suitable data type that has a dynamically determined number of dimensions. Surely someone has made a crate for such a data type.