avoid vanilla MC from creating new snow if already some there
- renamed some variables, removed unused variables.
- equalized number of layers: minimum is zero (1 layer), maximum is 7 (8 layers)
- always mask meta wiht 7 (meta & 7)
- restructured updateTick() so it is (hopefully) easier to understand

Still open:
- speed reduction is a bit digital:
  1 layer = no speed lost;
  2 layers = 2% slower;
  3 or more = 42% less speed
- canAddSnowCheckNeighbors is not working very well IMHO:
  if I have a block (1 high) on a flat plain, snow will
  accumulate on its top, but not on its base
- Options to tweak generation speed
Merge pull request #692 from CHeuberger/fixAccumulateSnow

Fix accumulate snow