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

[mrzzzrm]

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.