Comment 10 for bug 422511

I think this is a nicer solution than my previous patch. I removed trough-border from calculations for the slider rectangle (which is used for mouse detection) and instead add/subtract the correct values when drawing the slider.

For Firefox, I wanted to do something similar, modifying moz_gtk_scrollbar_thumb_paint() to draw a smaller slider, but otherwise using 0 for trough_border. I'm having problems implementing the latter, because even if I explicitly set metrics->trough_border to 0 in moz_gtk_get_scrollbar_metrics(), I can't see any difference.