Add matrix property to group

[wcandillon]

It could be useful to add a matrix property to group. The idea is that if you work with a transform library you may want to supply the matrix directly instead of decomposing it back to the sequence of 2d transforms.

We could task advantage to this small task to add support for robust Matrix construction via ::fromValue the same way we did it for points, rect, and rrect in Skia.

We could also think about some sort of lerp utility to go from one (random) transform to any other (random) transform.