not playing dangerous games with setLayerIndex

using setZIndex on drawing layers so order doesn't get messed up

A fix for #1134.

The behaviour before played quite a lot with the internal layer ordering in the openlayers array in the map object. This resulted in inconsistent behaviour when shuffling layers around. In this PR the only thing that changes on the overlaying drawing layers is the Z index, they'll still be demoted in the internal array, but this makes sure things don't go astray when shuffling around with layers.