Fixed GetModesARB implementation

The correct way to query number of available pixel formats is to use
Wgl.Arb.GetPixelFormatAttrib(NumberPixelFormatsArb), not
Wgl.Arb.ChoosePixelFormats. This fixes an issue where Intel drivers
would fail to report any pixel formats in GetModesARB, even when
WGL_ARB_pixel_format is supported.