WIP: Implement rotate_uncropped()

[sjaustirni]

Fixes #323

Implements rotate_uncropped(), which rotates the original image passed in and returns a new image, which contains the whole rotated original image centered in this new image - in other words; the function rotates the image and returns it uncropped.

Thank you @NyxCode for the original inspiration.

As always, if the regression test is undesired, it can be removed :)

[theotherphil]

Thanks for the PR. This is definitely a useful function to add.

The image you're committing for the regression test appears to have a black bar running down the left hand side. Do you know why this is?

Could you please also add a regression test for a more interesting angle of rotation.

[sjaustirni]

@theotherphil Good catch! My image viewer has blackish background, so I did not see it initially.

It seems that the black bars are more prevalent, there is also a horizontal one if you rotate by correct amount (see b84c1b587fad1df109e833dc53d455d4c84eab16).

I'll try to resolve it, but currently I do not know what the underlying issue is.