FontCache should cache font table blobs for harfbuzz

[burg]

Right now, every time text is drawn we create a new hb_blob_t for the font face's font table. This is expensive and unnecessary. We should instead store this data with each Font object. Rather than creating new font tables for each Font object (which can represent the same font at many sizes--with the same font table data), the tables can be reused when the Font is instantiated several times.

See http://dxr.mozilla.org/mozilla-central/gfx/thebes/gfxFont.h.html#l408 for Gecko's solution to this issue.

[burg]

For now, the Font instance reuses a Shaper, which reuses the harfbuzz resources. It may be possible to reuse these font tables across font instances, but that's a future optimization not worth thinking about right now.

[burg]

Followup issue for caching is #197.