Replace binpack_t with slot_image_t.

In most cases, this produces far better results than binpack_t. The
downside is that it consumes a lot more memory and is slower, but that's
not too important for the situation.

In a few odd cases, binpack_t will do better, like when packing very
thin elements next to each other, but this usually results in packing
oddities that result in more font pages rather than fewer.
Make frames REALs. Wrap ops in begin/end transaction.
Make the minimum glyph size the cell size.
Add --static to snow-common linking.
Use pixel-perfect packing.

Uses more memory (a lot more), but it does the job.