Reduce number of font instances and shaper structures created.

The font cache previously kept weak references to the fonts,
however in layout these fonts are dropped before they are
referenced again, so no caching was being used.

For now, just hold a strong reference in the cache, which means
that fonts will never be thrown out (this will need to be fixed
in the future).
Fix comment and and reference to bug #3300
Merge pull request #3298 from glennw/fix-font-cache

Reduce number of font instances and shaper structures created.