Implement Synthetic Bold Support

[Gankra]

Some fonts don't natively support bold, and force the browser to synthesize them. The synthesis is fairly simple: draw each glyph multiple times, with horizontal offset. In fact servo/gecko could just feed us multiple copies of each char to do it for us... except that wouldn't produce the correct result for transparent text. Requiring the browser to generate a stacking context with transparency would be wasteful, I think.

"synthetic bold" should be made a font instance key, as introduced in #1602

Sadly, I don't have a good synthetic-bold-needing example yet.

[Gankra]

Ah found an example by grabbing something from adobe's free font collection:

<script src="http://use.edgefonts.net/basic.js"></script>
<p style="font-weight: bold">
Native Bold
<span style='color: rgba(0,0,0, 0.5)'>
Native Transparent Bold
</span>
<span style='font-family: "basic"'>
Synthetic Bold
</span>
<span style='font-family: "basic"; color: rgba(0,0,0, 0.5)'>
Synthetic Transparent Bold
</span>
</p>

[Gankra]

Gecko+webrender today does everything right except the last one, because it sends us double glyphs to emulate the bold, but in that case expects transparency to be implemented by the DrawTarget.