gvRender*: render subgraph when given, not always root graph

gvRender*: render subgraph when given, not always root graph

These functions always rendered the root graph, even if a sub graph
was given. Apart from being incorrect, this caused
heap-buffer-overflow when trying to render a graph consisting of two
subgraph of which only one had a layout. Also, even if both subgraphs
had layouts, they somehow mixed up the bounding boxes of the graphs
causing some nodes and edges not to be rendered since they were
outside the graph bounding box.