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>
This (and the other `___OnTop` methods) should be removed - they are kinda useless seeing as they just delegate to other methods.