Fix hit testing for transformed elements

[nicoburns]

The correct way to handle hit testing elements with a transform applied is to:

• If the element has a transform, compute the inverse transform and store it
• When hit testing, apply the inverse transform to the pointer coordinates before testing the against the elements untransformed position.
• A transform with scale(0) needs to be special cased. In that case there is no inverse of the transform matrix (but the element is invisible so we simply return false from any hit tests).