Set materials for flowers and world debris so snow cannot form on top of them

The materials for some basic blocks were set strangely:

* Flowers (BlockFlower and BlockFlower2) were using the default material (stone).
* Debris (BlockWorldItem) was using the wood material.

I changed the debris items to use Material.circuits (like torches) and changed the flowers to use Material.plants.  This will prevent snow layers from forming on top of them.