Matrix4 decompose bug

There was a bug in Matrix4.decompose that would not properly account for a negative determinant matrix.  It would take the absolute scale vector, but if the matrix has a negative determinant, one of the scales needs to be negative (it doesn't matter which one) or the results are not a proper decompose.

BTW this technique of just inverting one of the components of the scale for negative determinant matrices is common, see last paragraph on this page: http://callumhay.blogspot.ca/2010/10/decomposing-affine-transforms.html

I wrote a unit test for Matrix4.compose/decompose that would fail without this fix and now it passes correctly.