Add possibility to set the number of zoom levels

By default, the number of zoom levels is 0 and mouse scroll won't do anything.
And you can set the number of zoom levels with setNbZoomLevels to enable the possibility to zoom with mouse wheel up to the defined number of levels.
Smarten the doRedraw function

The doRedraw function now add the missing layers and delete the useless ones.
It is way quicker than calling doRender.
Prevent drill across