theme_image_srcset() multiplier keys always equal zero in srcset

I think there is an issue with commit 3862ee3.

That patch replaced line 1578 in the theme_image_srcset() function with this:
$key = (int) ((float) drupal_substr($src['multiplier'], -1) * 100);

Which always equals zero (0) when run on a multipler like '1x', '2x', or '3x' because the drupal_substr() function always outputs the last character: 'x'. So the $srcsets array always overwrites itself at key zero.

I'm not really sure why the code was changed from what was there before which seems like the safest approach since it doesn't clash with the width key on line 1574:
$key = $src['multiplier'];