Fixed cave render for solid unknown blocks.

`is_transparent` returns 1 for every unknown block because that's an easy way to
make them ignore lighting. However, this makes cave render useless with a map
containing unknown ores.

The definition `is_known_transparent` checks whether a block is transparent and is known.

This is a workaround, a proper fix would be to have unknown blocks be neither transparent nor
solid. Or make them both. But don't prefer one of the two.

**NOTE:** This was not tested on a world containing unknown blocks, as I had none handy. But it didn't affect the output of exmaple.

Affects issue #982