Restrict width of progressbar in example-screenshot.js

As more and more examples will be processed with this script, the
progressbar would grow way too long. This commit makes the width of
the bar fixed. Every character in the bar now equals 5% progress,
regardless of the number of examples.

Before:

    Capturing 29 example screenshots.
    [>                            ]   0.0 % done
    [->                           ]   3.4 % done, anchored-elements.html
    [-->                          ]   6.9 % done, animation.html
    [--->                         ]  10.3 % done, bing-maps.html
    [---->                        ]  13.8 % done, brightness-contrast.html

After:

    Capturing 29 example screenshots.
    [>                   ]   0.0 % done
    [>                   ]   3.4 % done, anchored-elements.html
    [->                  ]   6.9 % done, animation.html
    [-->                 ]  10.3 % done, bing-maps.html
    [-->                 ]  13.8 % done, brightness-contrast.html