remove useless inheritance in OGLCombiner
[DemuxerCleanup] Still decrease the scope of DecodedMux class. In the futur, every queries about the combiner mode content (e.i.: What the ColorCombiner is intent to do) will directly use ColorCombiner class methods. I guess it should have worked like this but GPUs had less flexibility. OpenGL now provide enougth control that will make this queries mainly usefull for optimization purpose (e.g.: Texture updates).