flattening onto a temporary image to the QgsMapLayerRenderer subclasses
This avoids the generic QgsMapRendererJob class from having hardcoded
layer type dependent behavior, and instead moves that layer-type specific
logic to the subclasses designed for that particular layer
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
