Queues up a redraw of an actor and any children.
The redraw occurs once the main loop becomes idle (after the current batch of events has been processed, roughly).
Applications rarely need to call this, as redraws are handled automatically by modification functions.
This function will not do anything if this is not visible, or if the actor is inside an invisible part of the scenegraph.
Also be aware that painting is a NOP for actors with an opacity of 0
When you are implementing a custom actor you must queue a redraw whenever some private state changes that will affect painting or picking of your actor.