Matrix interpolation

> Might be clearer to call these perspectiveDeterminant and perspectiveInverse in that case.

These functions calculate the actual determinant and inverse of 3x3 matrices. I don't see the relevance of it only being used on perspective.

Determinant is used as an optimization in one place, but it's also used in inverse, which is used in calculating perspective. I think we should just delete the comment and leave the code as-is.