transform calculation issue causing gaps above image

[jacdx]

I'm having an issue getting the library to position the parallaxed image correctly in a responsive viewport. It gets a positive Y value in the matrix transform, even though it shouldn't.

I was able to reproduce the issue on the Github Page for this project. If you shrink your viewport to less than 1000 pixels wide, you'll see a gap above the image caused by this transform issue.

Chrome 64 / OSX