Multi-dimensional scale

[kvark]

We have vectors and we have ScaleFactor, which is 1D. What we are missing is an ability to scale vectors by a non-uniform scale with potential change of space. This would be helpful for code in servo/webrender#3046

[nical]

Scale2D<T, SrcUnit, DstUni> and Scale3D<T, SrcUnit, DstUnit> would make sense.