Povray _prefix type (const char* -> std::string)

I encountered a problem with the povray integration. The _prefix type was const char*, but my code flow first assigned the prefix in the constructor (only copied the pointer) and then went out of scope (the _prefix was pointing to empty memory). After that Povray initialisation was called and _prefix was empty. I changed that to std::string which implicitly copies the value, thus removing this problem.

Just a minor thing, but I think this could fix it.