TransformFeedback::setVaryings(): Take std::string instead of const char*

For the two convenience methods taking ```std::array``` and ```std::vector``` as parameters, change the template type from ```const char*``` to ```std::string```. Note that, e.g., ```{"o_position", "o_color"}``` can still be used as a parameter. Programs that are not hard-coding the names of their transform feedback varyings benefit from this change.