Extra-high res screenshots

[Olly Betts]

It would be useful to be able to render screenshots at a higher resolution than the screen, e.g. for publication.

OpenGL implementations typically support a larger res than the screen, but not hugely so (e.g. 4000x4000 on one of my machines). So we probably want to split the image into tiles and render each individually, then join them together. This would allow an image of almost arbitrary size (it might need to fit in memory, at least for the wxWidgets image support, but that's probably fine for modern machines - even A0 at 600dpi should only be a couple of GB).