Make flowing liquids "fall through" for water source nodes

This makes liquids in float and falling_node groups fall through
flowing liquid nodes instead of being supported by them in the air.

builtin/game/falling.lua

@@ -536,13 +536,17 @@ function core.check_single_for_falling(p)
 				local success, _ = convert_to_falling_node(p, n)
 				return success
 			end
+			local d_falling = core.registered_nodes[n.name]
 			-- Otherwise only if the bottom node is considered "fall through"
 			if not same and
-					(not d_bottom.walkable or d_bottom.buildable_to) and
-					(core.get_item_group(n.name, "float") == 0 or
-					d_bottom.liquidtype == "none") then
-				local success, _ = convert_to_falling_node(p, n)
-				return success
+			    (not d_bottom.walkable or d_bottom.buildable_to) and
+			    ((core.get_item_group(n.name, "float") == 0 or
+			      d_bottom.liquidtype == "none") or
+				(core.get_item_group(n.name, "float") > 0 and
+				 d_falling.liquidtype == "source" and
+				 d_bottom.liquidtype ~= "source")) then
+			    local success, _ = convert_to_falling_node(p, n)
+			    return success
 			end
 		end
 	end