WGL: Get rid of the use of the Host_GetRenderWindowSize() call.

Just use the Windows API to accomplish the same thing (this is what is done in Update()).

This makes the backing window handle the correct data-type for Windows for easier use in function calls.
AGL: Remove the call to Host_GetRenderWindowSize().

Just uses platform-specific ways to do the same thing.
X11_Util: Get rid of the call to Host_GetRenderWindowSize().