Proper tab stops for preformatted text

[mbrubeck]

Currently we advance by eight spaces for each tab character ('\t', U+0009) in preformatted text: https://github.com/servo/servo/blob/master/components/gfx/text/shaping/harfbuzz.rs#L514

Instead, tabs should advance to the next multiple of the tab-stop width:

All tabs (U+0009) are rendered as a horizontal shift that lines up the start edge of the next glyph with the next tab stop. Tab stops occur at points that are multiples of 8 times the width of a space (U+0020) rendered in the block's font from the block's starting content edge.

— http://www.w3.org/TR/CSS2/text.html#white-space-model