Cleaning stuff

+Removed dependency of NzNode from space coordinates. Uses integer logic
instead. Consequence : it is now feasible to set a global normal to the
entire terrain, to orient it in any direction. First step towards
DynaPlanet
+NzPatch is now a bit lighter and makes less calls
+Centralized vertex position and height computation in NzQuadTree
+Overall code cleaning, made things easier to maintain
+Updated shaders to remove an artifact