Now that MainNoGUI is properly architected and GLX doesn't need to
sometimes craft its own windows sometimes which we have to thread back
into MainNoGUI, we don't need to thread the window handle that GLX
creates at all.
This removes the reference to pass back here, and the g_pWindowHandle
always be the same as the window returned by Host_GetRenderHandle().
A future cleanup could remove g_pWindowHandle entirely.
Required the removal of EGL.h from EGL.cpp.
Removed the similar includes from AGL.cpp, GLX.cpp, and WGL.cpp to retain consistency.
All GL interfaces are now centralized on GLInterface.h
