[1.4.7] Fix grass growth broken by jad rename compile fix. Fixes #616.

The jad-style variable rename in 1.4.7 (for preparation for the 1.5
update) introduced some variable collisions; they were fixed
(build 245) but an incorrect variable name was used in BlockGrass,
causing the grass growth condition to never be true. This fix
uses the correct variable for the coordinate opacity check.