This commit adds a depth map to the frame so we can find the "top most"
opaque layer by stacking order for a given point.

This lets us determine whether the element being hovered was part of the
given layer in the previous frame, which fixes the hover issue.

Still to do:
* [ ] Fix up mouse events too (I tried this, but it's not working yet,
and I want to understand why)

Release Notes:

- N/A