Do we really need two formats?

I added the output format 'png' and 'text'.

So we need to reduce the two hook_graphapi_formats back to one? Let's see what happens.

I think the theme function can nice decide what to output. But this depends on the needed theme functions.

Say one want to theme each node by adding an background image.

I've reverted the text_output.

We now have on engine with one format 'graphviz'.

graphviz config has it's own output settings and format.