nested: Add a ‘renderer’ mechanism with a vtable

nested: Add a ‘renderer’ mechanism with a vtable

Eventually the nested compositor example will want to be able to cope
with either rendering as it does now with a blit to an intermediate
surface or by attaching the client buffers directly to a subsurface
without copying. This patch moves the code that is specific to the
blitting mechanism into a separate set of functions with a vtable to
make it easier to add the second way of rendering in a later patch.