photon::transform additions

[volbot]

Changes:

1. Added transform::{shearx, sheary}; added example (examples/shear.rs)
Note: The change to image size is a lot more drastic than before, and pretty much any usage/implementation will need some kind of cropping function.

2. Rewrote transform::rotate to use Paeth rotation; added interpolation; added example (examples/rotate.rs)
Resolves: #170 #178
Note: Slightly less performant than previously. This could hypothetically be faster if all three shears were incorporated into a single loop, but the extra considerations needed in order to properly interpolate made the gains paltry in every case I tried.

3. Changed signature of transform::crop to not require a mutable reference
Resolves: #177

[volbot]

I just removed the content-aware crop, ultimately leaving the cropping of the resultant image to the user (in line with previous behavior). After some personal testing, I realized that it sometimes cuts too aggressively, especially on small images with concave sections of alpha. Other than that, everything appears to work correctly.

[volbot]

I just fixed the Rustfmt issues and synced the recent multiple.rs changes to the branch, so it should(?) be mergeable now.

[silvia-odwyer]

@volbot It appears the rustfmt test isn't passing, could you run cargo fmt on the codebase? It will do any other additional formatting required, and I'll then be able to merge the PR ✅

[silvia-odwyer]

@volbot That's great, thanks very much again for your excellent contribution! It's greatly appreciated. I'm delighted that the library will have the new rotation implementation, as well as the added shear functions. 🎉 Everything looks good to me, so I'm going to merge this ✅