Fix for cursor problem in samples/simple-editor.rb

samples/simple-editor.rb seemed to show an off-by-one the cursor.  In fact, the cursor was positioned right, but just lagging on redraws (i.e. cursor moved after drawing for it had already been done)

We update position only as needed and lean on RedrawingAspect for redraws.

cc/ @PragTob to make sure my use of `RedrawingAspect` here is correct, and @wasnotrice since #503 might have a merge conflict if this is landed first (there are a few textcursor painter changes happening that PR).