Implement Transformation::stretch, Transformation::inverse and the safer Transformation::try_inverse

[airstrike]

Transformation::stretch seems like a non-issue since it's just scaling across a single axis. Useful for custom widgets that want to scale in that fashion, such as anything with a timeline: DAWs, video editors, charts...

While we're on the topic of Transformation, it can incredibly handy to be able to invert some previous complex transformation so we can go to-and-from different coordinate systems, so I think it would be helpful to add some inversion method. After iterating for a bit, I am still not sure about the API for inverse and try_inverse.

I have tests to add if you think it would be helpful.

[airstrike]

@edwloef was kind enough to point out that Transformation::scale_factor needs to account for the possibility of non-uniform scaling transformations. I've changed its return type to (f32, f32) and worked through the compiler errors.

It's looking good, but I want to do some more testing, likely by modifying the custom_quad example to help visualize it.

I just don't really have the time to wrap this up this week so I'll have to come back to it later.