This function provides a convenient way of doing this. A container, when it receives a call to its
draw function, calls propagate_draw once for each child,
passing in the
cr the container received.
propagate_draw takes care of translating the origin of
cr, and deciding whether the draw needs to be
sent to the child. It is a convenient and optimized way of getting the same effect as calling
draw on the child directly.
a child of this