Expose z-value for FNode and use it in FSprite.

This simple addition allows doing parallax/varying-depth layers with a perspective camera.

Signed-off-by: Patrick Hogan <pbhogan@gmail.com>
Rename to z to meshZ to avoid confusion.

Signed-off-by: Patrick Hogan <pbhogan@gmail.com>
Merge pull request #173 from pbhogan/feature/expose-node-z-value

Expose a z-value for FNode and use it in FSprite.