this is a bad fix, but it doesn't matter a whole lot. The problem stems from furnace not having a valid 0 variant, so things go haywire when the default mapped 0 variant is registered to LIT_FURNACE because the default state is of course unlit.
This is a major change to the way block metadata is handled within the PM core. This separates variant metadata (which really ought to be part of the ID) from state metadata, and in a couple of cases flattens separate states of blocks together.
The result of this is that invalid variants can be much more easily detected, and additionally state handling is much cleaner since meta is only needed at the serialize layer instead of throughout the code.
