Match affine3d skews {_xy, _xz, _yz} up to affine2d_skew behaviour

[geoffder]

Currently, affine2d_skew and affine3d_skew_xy have different behaviour.

include <BOSL2/std.scad>

sq = square(10, center=true);
m2 = affine2d_skew(45, 0);
m3 = affine3d_skew_xy(45, 0);

linear_extrude(1) polygon(apply(m2, sq));

translate([0, 0, 5])
    multmatrix(m3)
        linear_extrude(1) 
            polygon(sq);

Similarly, affine3d_skew_xz and affine3d_skew_yz do not skew the same way as affine2d_skew does (if you were to look down the third axis at the relevant plane), though I think that they should be based on the documentation (as in, skewing along the plane corresponding to the arguments).

With these changes, the code above now provides similar results for 2d and 3d xy skewing:

Apologies if I've misunderstood something, but I thought I should submit a patch in case.