use middle pixel row for vertical uploads

[jywarren]

This is a tough one -- we currently (on this line) auto-select the brightest row on "mobile" uploads, which is based on the user-agent including the letters "Mobi" -- which may not be good anymore. This can be tested by adding ?m=true to the URL to fake mobile.

Should we auto-select brightest? This'll often be blown out.

Should we use the middle line? This may not be good for all devices, but it'll probably have a higher success rate than the top row, as in the live capture data.

Should we put a "change" button on the "cross section line"? Then, although we could get it wrong, it'd show up for users and be more explicit, and more obvious how to change it.

[jywarren]

Also keep in mind that if brightness is very low, this is displayed: