Retrieves a Cairo context to be used to draw on the `GdkSurface` of
A call to [method@Gdk.DrawContext.begin_frame] with this
context must have been done or this function will return
The returned context is guaranteed to be valid until [method@Gdk.DrawContext.end_frame] is called.
a CairoContext that is currently drawing
a Cairo context to be used to draw the contents of the `GdkSurface`. null is returned when