Multiple camera system.
Implemented rendering order.

Now the elements to render are added to a list where are ordered using its parallax index. A lower index indicates a that must be renderer first, and a higher index to render later, so objects who are nearer to the camera will be renderer later and will be over those with lower parallax index.
Changed rendering order from parallax index to z-index.

Now the rendering order is set by the z-index variable and the parallax index only controls how much the camera movement affects the position of an object.

This abstractions let us to make an static image (parallax index = 0) which can be renderer over all the other objects (high z-index, ie: 99). This can be used for UI elements, for example. It was imposible to archive this with the previous implementation.