Previously, any font template that could not be found would return
an item from the last resort font family. Now, the last resort font
family is only searched if the entire list of supplied font families
is empty.
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).
