When performing polar decomposition in 2D as B = R.S, where R is rotation (with determinant +1) and S is scaling, use the convention that reflections are absorbed into S through a reflection around y axis.
In 3D, this is done by using a reflection along all three axes, but since the dimensionality is even in 2D, one axis needs to be chosen.
Fixes Matrix32::get_rotation and Matrix32::get_scale (which weren't properly fixed in #7445).
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