warn if PAGE regions overlap

[bertsky]

IIUC this is intended for pixel classifier training, where a flat colormap is the correct representation. But in PAGE-XML, regions may overlap each other arbitrarily, and can also be recursive (and in some cases must be).

So to avoid any unintended errors, IMHO users should be warned whenever overlaps do exist (and if they do, how large).

Also, there should still be a reasonable fallback strategy. But in the current implementation, images replace text, separators replace images, tables replace separators. For general purpose layouting, I think this should instead be image < text < table < sep.