Fix alignment issue when pictures do not fill the pictures-group line

[QSchulz]

When pictures are too small to fill a pictures-group's line (which can happen with two portrait pictures; for example on a 1080p screen since the pictures are x600 thumbnails, only two portrait pictures in one line would make the content of the line ~1200px instead of the ~1920px (it's actually with the current syle.css 77% of that => ~1500px) of available space.

The issue is particularly present when zooming out:

I'd expect the pictures to fill the whole line.

With the fix:

Fixes #92

[JocelynDelalande]

@QSchulz Thanks for your patch :-) Does it work for lines with a lonesone picture, fixing the bug mentioned here ?

[QSchulz]

@JocelynDelalande I still have to fix this Travis issue, hopefully this week-end it's resolved.

Yes it does, but it's not pretty for two reasons:

• the image is basically upsized so it gets pixelated at one point,
• (this one's not pretty according to my taste) if the image is in portrait mode, the image is now taking the full width of the window and since the ratio is preserved, the image height might even be bigger than the window's. e.g.: (this is a screenshot of the full page on a 1080p screen with a one-image line)

[beudbeud]

@QSchulz is look have problem with the option 'select_streams'

Failed to set value 'v:0' for option 'select_streams'

Have you try with ffmpeg and avprobe?

[beudbeud]

@QSchulz have try in python 3? because on mine computer i get error