Detect upside-down images

[danvk]

This obviously breaks OCR:

→ 7w Hupaouc ''soav (iaectg pts MaoV aeN ussE teowloc 'Teeag sntunC

An example is 705124b. The thumbnail can't save me here, unfortunately; it's also rotated 180°.

The first step is to estimate how common this is. But writing a detector for upside-down text should be easy -- generating training data is as simple as flipping good images upside-down!

[danvk]

I stepped through 100 portrait images and found one upside-down image (727002b). So perhaps ~1%.

[danvk]

I'm unlikely to do this algorithmically. Users can (and do) rotate & hand OCR these images.