Matrix interpolation

> I think this comment was copied from web-animations.js, and it's not 100% correct. The function is used on perspectiveMatrix, which is the portion of the matrix that includes a perspective component. The comment is justification for ignoring the third comment in the determinant calculation.
> 
> Maybe the best bet is to rename the function to decomposePerspectiveSubMatrix?

Hmm? Doesn't that comment only apply to determinant()?