Existing code drawing on a GDK window has to handle the direct drawing
and the buffered drawing by itself, by checking the window type and
whether or not the window is backed by a native windowing surface. After
that, the calling code has to create a Cairo context from the window and
keep an association between the context and the window itself.

This is completely unnecessary: GDK can determine whether or not it
should use a backing store to draw on a GdkWindow as well as create a
Cairo context, and keep track of it.

This allows to simplify the calling code, and enforce some of the
drawing behavior we want to guarantee to users.

https://bugzilla.gnome.org/show_bug.cgi?id=766675