resolve_fonts() is sometimes called for the same asset with different
lists of things to check. If it is called first with a font, and
then without one, we want to keep the font from the first call.
This is simpler and allows us to remove the hack of allocating some
"maximum" buffer for incoming JPEG2000 data.
It does mean that the buffer is zero-initialized before being written
to, but hopefully that doesn't matter too much.
