Auto merge of #1240 - kvark:font, r=glennw
Auto merge of #1240 - kvark:font, r=glennw

Unix font rasterization follow-up

Follow-up to #1220
I found the way I handled `bitmap.width` to be incorrect. The new code is hopefully simpler.
r? @glennw
(this is yet to be confirmed about resolving the [intermittents](https://github.com/servo/webrender/pull/1220#issuecomment-300694402))

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