Improve Background cache when changing papersize

[ThomasParistech]

Instead of re-downloading the whole map when the papersize slightly changes, it would be better to download the area enclosing all possible papersizes and then extract parts of it

Switching between papersizes would much faster

[JulesL2]

@ThomasParistech You should check what I have done in get_max_download_bounds_across_layouts in BaseVerticalLayout class. It is the same idea but with all paper sizes instead of all layouts for a single paper.