[css-transforms] Coordinate inversion for 3D matrix decomposition

[kevers-google]

Link to spec: https://drafts.csswg.org/css-transforms-2/#decomposing-a-3d-matrix

In cases where the determinant is negative, the spec proposes inverting all scales and matrix elements. Though this leads to a valid decomposition it means than any 3D decomposition of a 2D transformation matrix with a scale inversion results in a decomposition that cannot be expressed as a sequence of 2D transform operations.

Consider the 3D decomposition of matrix(-1, 0, 0, 1, 0, 0).

This is equivalent to scaleX(-1); however, following the recipe, the 3D decomposition is equivalent to scale3d(1, -1, -1) rotateX(180deg).

3D decomposition of a 2D transformation matrix should yield comparable results to the 2D algorithm outlined here: https://drafts.csswg.org/css-transforms/#decomposing-a-2d-matrix. This can largely be addressed by using a similar strategy to the 2D case and only invert 1 scale when the determinant is negative. The 2D spec proposes flipping the axis with the minimum unit vector dot product. This approach should work for the 3D case as well.