Biome API decorations: 'spawnby' searches a 3D neighbourhood

The neighbours checked are the 8 nodes horizontally surrounding the decoration base
and the 8 nodes horizontally surrounding the ground node below the decoration

doc/lua_api.txt

@@ -3331,7 +3331,9 @@ Definition tables
     --      ^ Number of nodes the decoration can be at maximum.
     --  ^ If absent, the parameter 'height' is used as a constant.
         spawn_by = "default:water",
-    --  ^ Node that the decoration only spawns next to, in a 1-node square radius.
+    --  ^ Node that the decoration only spawns next to.
+    --  ^ The neighbours checked are the 8 nodes horizontally surrounding the lowest node of the
+    --  ^ decoration, and the 8 nodes horizontally surrounding the ground node below the decoration.
         num_spawn_by = 1,
     --  ^ Number of spawn_by nodes that must be surrounding the decoration position to occur.
     --  ^ If absent or -1, decorations occur next to any nodes.

src/mg_decoration.cpp

@@ -246,7 +246,7 @@ bool DecoSimple::canPlaceDecoration(MMVManip *vm, v3s16 p)
 		return true;
 
 	int nneighs = 0;
-	v3s16 dirs[8] = {
+	v3s16 dirs[16] = {
 		v3s16( 0, 0,  1),
 		v3s16( 0, 0, -1),
 		v3s16( 1, 0,  0),
@@ -254,7 +254,16 @@ bool DecoSimple::canPlaceDecoration(MMVManip *vm, v3s16 p)
 		v3s16( 1, 0,  1),
 		v3s16(-1, 0,  1),
 		v3s16(-1, 0, -1),
-		v3s16( 1, 0, -1)
+		v3s16( 1, 0, -1),
+
+		v3s16( 0, 1,  1),
+		v3s16( 0, 1, -1),
+		v3s16( 1, 1,  0),
+		v3s16(-1, 1,  0),
+		v3s16( 1, 1,  1),
+		v3s16(-1, 1,  1),
+		v3s16(-1, 1, -1),
+		v3s16( 1, 1, -1)
 	};
 
 	// Check a Moore neighborhood if there are enough spawnby nodes