API: gdk: Remove ability to render window contents

API: gdk: Remove ability to render window contents

The functions gdk_pixbuf_get_from_window() and
gdk_cairo_set_source_window() are unreliable and depend on the windowing
system (they work great on X11 and Win32, less so on Quartz and Wayland).

With the switch to new drawing API and OpenGL, we can definitely no
longer support a generic way to snapshot windows.

People should either write windowsystem-specific code or draw their
widgets directly - like with gtk_widget_draw() - if they need to get a
rendering.