Fix for WinGL initialising GraphicsCapabilities

Seems that 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 getting of graphics extensions to their own method and put it in the constructor with other 'GL.GetInteger' calls rather than `Initialise()`
Merge pull request #1260 from Aranda/fix_wingl_graphicscapabilities

Fix for WinGL initialising GraphicsCapabilities