add support for a different default renderer

I recently wanted to use something else than OpenGL in OF and didn't see a way to not automatically initialize the opengl renderer and glew. This PR introduces different versions of ofSetupOpenGL so a default renderer can be passed and an optional bool whether to initialize glew or not. Personally I think this function is now a little bit too long. But ways how  an OF app could also be initialized is a completely different topic...