Sided block opacity.

Implement isBlockSolid and isBlockSideSolid, as well as isSolid and isSideSolid on BlockType.
To be used with Torches, Levers and other blocks that are side surface dependent. Will add more code regarding this functionality later when I need it.

Josh Lloyd <j.nevercast@gmail.com>
That was their intention, it made code elsewhere in the project look more tidy by reducing arguments and increasing clarity. But removing these is no issue.