Split SceneOpenGL::Window into specific classes for OpenGL 1 and 2

The Window implementation performed many checks whether the rendering
uses the OpenGL 1 or OpenGL 2 code path and there were quite a few
cludges around to make this work.

So instead of many if-else blocks the specific code has now been moved
into a specific sub class and calls to pure virtual method in the base
class are used to trigger this behavior. Although that adds some overhead
in a rather hot code path it should be better than the many chained
method calls used before to handle OpenGL 1 and 2.

It also makes the code a little bit more readable as all the complete
OpenGL 1 implementation is now in one block ifdefed for OpenGL ES.