[css-ruby] Default line-height of <rtc> should be zero

Since the font-size is applied to <rt>, <rtc> will still have a line-height
which is double of the font-size of <rt>. If there are multiple annotations
stacked, there will be a gap between levels because of the line-height. The
leading is also affected by this.

I propose that in the default style sheet, "line-height: 0;" should be
added to the rule for <rtc> so that there won't be a gap by default, but
authors can customize it if they want.

- Xidorn