map layer renderers
These renderers can then use the layer rendering time for the previous
map render job to refine their logic regarding when its appropriate
to start compositing the layer render
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!
