Use FT_Outline_EmboldenXY

Do we want version macro checks?

* We could raise the minimum version requirement.

* We could put this change off until such a time when we’re happy to raise the minimum version requirement. I didn’t actually notice any difference in the output, although I only did one quick test.

Do we want a runtime version check? Older FreeType is ABI-compatible but doesn’t have this function.

(Speaking of performance, the new version has to be twice as fast at emboldening. But I have no idea how slow or fast emboldening is to begin with.)