Improve font layout in Linux / Freetype platforms.
This patch contains two small changes:
* Fix the font size calculation that is passed to Freetype. This
now matches exactly how Webrender and Gecko calculate font size
to pass to layout.
* Enable light hinting by default for fonts when using Freetype.
We should make this configurable in the future, but this is a
better default than no hinting (and matches what most Linux
distros default to).
These two changes (along with the pending WR update) fix a lot
of the font layout issues on Linux. There is still at least one
remaining issue with hidpi displays on Linux that will be fixed
in a follow up patch.
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