Add a 2D scale+offset transform type

[nical]

This PR moves the corresponding type from WebRender and adapts it to euclid's convention.

Scale-offset transformations are common enough for a browser engine or UI toolkit that it is worth having a special case for. In addition to requiring fewer operations than Transform2D, ScaleOffset2D fits in only 4 scalars, and is guaranteed to be able to transform boxes and rectangles exactly (Transform2D can only produce conservative outer boxes).

[mrobinson]

Nice to see this! In Servo we also reuse WebRender's FastTransform, which might also be a useful abstraction for euclid as well: https://github.com/servo/webrender/blob/e1c924ebad9ffdfe8c8c606aba77eb3f888c396a/webrender/src/util.rs#L1007

[nical]

Added #547 to workaround the CI failure that comes from upstream.

In Servo we also reuse WebRender's FastTransform, which might also be a useful abstraction for euclid as well

I really want to keep euclid super-duper API-stable because of how much churn breaking changes create downstream. I feel like ScaleOffsetTransform2D is pretty safe to put in the API, but it's less clear for FastTransform in my opinion (there is a fair amount of nuance about how it could be implemented). Maybe we could add a separate "euclid_extras" crate where we put code that we would like to share or experiment with, but without the stability guarantees of the main crate?

[mrobinson]

Exposing a new crate seems fine, or perhaps we can just add this type to the webrender API upstream. Both are fine for the purposes of Servo.