Automatic cropping of snapshots

[thrust26]

The idea is to automatically remove all black bars at the top and bottom. And maybe the left and right too.

[sa666666]

Since I'm working on the PNG snapshot stuff ATM, I have some questions on this. First one is, why? What is the use case for this? Secondly, how would we detect the black areas? Right now the snapshot code is using Common::Rect to get the image from any larger surface (like in fullscreen mode). It would be easy enough to have another Rect that 'cuts out' the black area, but only if we can properly detect that area. IOW, applying a crop is easy; determining what to crop not so much.