(re)load the page for each screen size

[mgeier]

~For now, this is just an experiment, I don't know yet if it will work as intended.~

I don't know how the handling of progress should be adapted.

[mgeier]

OK, this works as expected.

Maybe I should explain:

My insipid theme shows a sidebar by default if the page is opened on a huge screen. However, it is not shown when opened on a small screen and the window is made larger afterwards.

Since the screenshot generation works like the latter, the large insipid screenshot doesn't look like it would actually look on a large screen.

[mgeier]

To make the situation clearer, here's a screenshot before this PR:

... and here's how it looks like after this PR:

Note that the huge screen is supposed to show the left menu-bar by default, while the medium and small screens don't show it.

[mgeier]

Thanks!