Add an option to mask everything not on the page or display it with an alpha effect

Sometimes we have some objects that are bigger than the page area (e.g. pictures), or objects we want to keep but not in the page (e.g original for clones).

We can put the page borders above the drawings but sometimes hidding or using an alpha outside the page would help to know what will be redered more easily.

An easy way to do it would be, for all pixels outside the page, to blend it with the background color using a factor.