This is an important change: new class (QgsMapLayerRenderer) is introduced
and it keeps all information from layer which is necessary for rendering.
Thanks to that, any changes to map layer will have no impact on rendering that
may be currently underway: if the user changes renderer, rendering will
not crash because it is using a different instance.
Work in progress: only vector layers, no labeling, no diagrams, iterator
still uses some bits from QgsVectorLayer.
Another change: QgsFeatureRendererV2, QgsSymbolV2 do not get access
to QgsVectorLayer - only to fields they need. Point displacement renderer
did more extensive use of QgsVectorLayer - currently it is not functional.
