Fix for WinGL initialising GraphicsCapabilities

Seems that my last pull request broke WindowsGL, and possibly Mac and Linux too. For some reason calling `GL.GetInteger(GetPName.MaxTextureSize, out maxTextureSize);` from a static method causes a null reference exception in OpenTK. I've worked around it by caching the _maxTextureSize in Graphics device. I've also move the parsing of GL extensions to their own method and moved it to constructor after the other 'GL.GetInteger' calls.