Single "Renderer" Class.

[xinyazhang]

Currently each SceneObject inherits QOpenGLFunctions_3_3_Core, but this is wasteful.
Try to refactor our current code to something like this:

Renderer --------(Read)--------> SceneObject
+---------- Render according to data from SceneObject.

[tycheng]

I created a graphics device class and pass that to every scene object. Right now there is only one instance of GLCore

[xinyazhang]

Current approach is not exactly what I wanted.
SceneObj should only provide data and there will be another Renderer class actual calling the GL functions with the data provided by SceneObj.

By anyway, do whatever you like.

[tycheng]

The reason I did the other way is that there might be variations of how scene objects need to be rendered. For example, I use glDrawArray for billboards, and use glDrawElements for general models and scenes. I understand what you said in the issue, but I felt this might become a little bit too restricted.