Affine transforms can't scale up

[CryZe]

If you use anything outside -1 to 1, it fails to convert the affine transform. This almost seems intentional, but I can't find the justification for it anywhere and it seems unnecessary too, as all other renderers I've tried don't have any such limitation.

https://github.com/google/forma/blob/fcef3e29ce87219eefaaf87ad5363f8335114b71/forma/src/math/transform.rs#L117-L125

[dragostis]

Layer transforms are not supposed to scale up, only scale down, translate and rotate. For everything else, you'll have to transform the Path instead.

The distinction here is made because translating the layer is cheaper, if it fits the definition above. We should add pointers to this in the docs.