Fix progress bar radius for opera.

`overflow:hidden` and `border-radius` doesn't work in in my OPERA (version 12.16, build 1860, platform linux).

There is an [example](http://jsfiddle.net/6ee3R/1/) from [stackoverflow](http://stackoverflow.com/questions/17087645/overflow-hidden-with-border-radius-not-working-as-expected-in-opera).