GDK-Win32: Reject WGL context if shaders aren't supported

GDK-Win32: Reject WGL context if shaders aren't supported

When we initialize OpenGL, check whether we have OpenGL  2.0 or later; if not,
check whether we have the 'GL_ARB_shader_objects' extension, since we must be
able to support shaders if using OpenGL for GTK.

If we don't support shaders, as some Windows graphics drivers do not support
OpenGL adequately, notably older Intel drivers, reject and destroy the GL
context that we created, and so fallback to the Cairo GSK renderer, so that
things continue to run, albeit with an expected warning message that the GL
context cannot be realized.

Also, when we could not make the created dummy WGL context current during
initialization, make sure that we destroy the dummy WGL context as well.

Fixes issue #4165.