better handling of unknown blocks on client

src/mapnode.cpp

@@ -138,6 +138,18 @@ void init_mapnode()
 			f->tiles[j].material_type = initial_material_type;
 	}
 
+	/*
+		Initially set every block to be shown as an unknown block.
+		Don't touch CONTENT_IGNORE or CONTENT_AIR.
+	*/
+	for(u16 i=0; i<=253; i++)
+	{
+		ContentFeatures *f = &g_content_features[i];
+		f->setAllTextures("unknown_block.png");
+		f->setInventoryTextureCube("unknown_block.png", "unknown_block.png", "unknown_block.png");
+		f->dug_item = std::string("MaterialItem ")+itos(i)+" 1";
+	}
+
 	/*
 		Initialize mapnode content
 	*/