Optional resizing without preservation of aspect ratio

[cmaggard]

Hello, first off I wanted to thank you so much for the implementation- playing around with this has been my first real stepping stone into Rust, and you've made it pretty easy!

During my tinkering, I've been trying to use some local images I had laying around, and noticed that some of them would silently fail. Namely, the ones that weren't already square. The images crate provides a resize_exact method for DynamicImage that works for resizing these in a local vendored copy of the code- do you think it would be worth adding an option to use it? If so, I could probably contribute a PR. If not, I could also contribute a PR updating the documentation to specify that only 1:1 AR images should be used. Either way, thank you!