The image class has a strict internal representation so we dont need "format".
PixelFormat enum can be passed to functions that modify the pixel data (like setPixels()) so that a proper convertion can be done.
We don't want to expose any OpenGL functions to client code. Because if we do, there is a chance we break client code if we switch implementation (Direct3D).
When writing the X11 (linux) implementation there was a problem with X11 defining a "Display" type and we also have a Display class in the engine.
So to fix that problem and minimize the risk for running into other name conflicts. We move everything from global namespace.
