Fix padding update
Fix padding update

`this.padding` was set once at the Mark creation and was never updated
on padding change in the Model. We now always use the current padding
value contained in the Model when it is needed.

SylvainCorlay/bqplot,ChakriCherukuri/bqplot,dmadeka/bqplot,dmadeka/bqplot,SylvainCorlay/bqplot,ChakriCherukuri/bqplot,bloomberg/bqplot,ChakriCherukuri/bqplot,SylvainCorlay/bqplot,bloomberg/bqplot,bloomberg/bqplot