Improve alt text of snapshot thumbnails
Improve alt text of snapshot thumbnails

The default alt text for these images was the filename, which is just a
bunch of letters and numbers. This wasn't helping anyone. Using the
title and viewport width is much better and will benefit people using
assistive technologies.

kalw/diffux,diffux/diffux,diffux/diffux,kalw/diffux,kalw/diffux,diffux/diffux