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.
Merge pull request #988 from CounterPillow/ftb-cave-fix

Fixed cave render for solid unknown blocks.
Refactored stairs rendering to support corners