Add CanChangeDirtToGrass function to Block Handlers.

src/Blocks/BlockSlab.h

@@ -97,6 +97,12 @@ public:
 		return "";
 	}
 
+
+	virtual bool CanChangeDirtToGrass(BLOCKTYPE a_Block, NIBBLETYPE a_Meta) override
+	{
+		return (a_Meta & 0x8);
+	}
+
 	
 	/// Returns true if the specified blocktype is one of the slabs handled by this handler
 	static bool IsAnySlabType(BLOCKTYPE a_BlockType)