Convert figure to png.

Figures are to be converted to png so that all browsers can read them,
however svg files are be kept to allow futher editions of the figures.

doc/additionalDoc/package.h

@@ -124,7 +124,7 @@ The program \ref dgshell_doc can be used to have scripting access to the dynamic
 library, where you can execute scripts and commands, load plugins, create entities and connect signals.
 
 Here is a typical use case for programmers:
-\image html figures/use-case.svg
+\image html figures/use-case.png
 
 \section References
 \anchor Mansard2007