paint method is already purely virtual
QPainter is enough
factor out ScreenLayer from GeometryLayer
wip idis
GeoGraphicsScene
always create a canvas image in texture mappers
GeoPainter::mapTexture()
const'ify
remove ViewportParams parameter from MarbleGraphicsItem::paintEvent()
const'ify changeViewport()
const'ify GeoGraphicsItem::paint()
WIP: split LayerInterface::render() into paint() and setViewport()