Flesh out Font and FontShaper APIs

[burg]

We need to port some of the font and shaper architecture from Gecko.

• Each Font object represents a fixed font face + style combination; it stores any platform-specific blobs; and has an as-needed FontShaper instance.
• All harfbuzz blobs should be reused across shaping calls, and owned by a Shaper object (which is owned by the Font instance). Right now, all shaping functions are static (in src/servo/text/shaper.rs), and create/destroy harfbuzz blobs and buffers per every single draw command.
• Native fonts can optionally provide GetGlyph/GetGlyphHAdvance callbacks, in case harfbuzz can't figure it out or there are platform-specific hinted widths available.