Transforms the given cairo context
cr that from
widget’s window is not an ancestor of
modification will be applied.
This is the inverse to the transformation GTK applies when
preparing an expose event to be emitted with the
signal. It is intended to help porting multiwindow widgets from
GTK+ 2 to the rendering architecture of GTK+ 3.
the cairo context to transform
the widget the context is currently centered for
the window to transform the context to