Add generalizated convolution filter

[vsuryamurthy]

At the moment, there is only a filter3x3 available. I would like to add a function which is a generalization of filter3x3. Here is the PR: https://github.com/image-rs/imageproc/pull/517

[willhansen]

I thought it was odd I couldn't find this feature

[theotherphil]

This is available as Kernel::filter

[ripytide]

I think one of the reasons it might not have been spotted is because it is an associated method to the Kernel struct which is rather inconsistent as nearly all image operations are free-standing functions.

I think making that function free-standing would be a good idea.

[theotherphil]

I’d accept a PR to make that change.