Added Block and id/data overloads to canPassThrough, centralTopLimit and isNaturalTerrainBlock.
Made everything that uses canPassThrough use data values.
Made /ascend and /descend use centralTopLimit.
BlockType adjustments

- Flagged all snow layers except single layers (data values 0 and 8) as "cannot be passed through".
- Fixed centralTopLimit for trap doors and fence gates and cake blocks with data values 6..15.
Gave the function containing the code for the //gmask command a better name.