remove misplaced label "0°S" in Decimal case in the west (reverts part of commit 3a781acb5d35691e7b2232498c5b6d6d7903bca4)
avoid overlapping of longitude labels in some cases (regression of commit f1b4db185111e9c23a7e7338176fee3171cf48de)

When the whole earth was visible, the lower longitude labels in Degree mode were overlapping.
This patch restores the old if clauses since the new ones (itStep != 0.0 || itStep != 180.0) always evaluate to true for any itStep.
Allow for labels in GeoLineStringGraphicsItem.
GraticulePlugin: Use GeoGraphicsItems for rendering the grid
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()
remove customPaint()
WIP: split LayerInterface::render() into paint() and setViewport()
deactivate to-be-ported RenderPlugins