Fix Z Index on platforms in 2D platformer demo (#927)

2d/platformer/level/level.tscn

@@ -229,6 +229,7 @@ position = Vector2(16, 0)
 [node name="Platforms" type="Node" parent="."]
 
 [node name="Platform" parent="Platforms" instance=ExtResource("3")]
+z_index = 0
 position = Vector2(707, 400)
 
 [node name="AnimationPlayer" type="AnimationPlayer" parent="Platforms/Platform"]
@@ -239,6 +240,7 @@ libraries = {
 }
 
 [node name="Platform2" parent="Platforms" instance=ExtResource("3")]
+z_index = 0
 position = Vector2(-47, 80)
 
 [node name="AnimationPlayer" type="AnimationPlayer" parent="Platforms/Platform2"]
@@ -249,7 +251,6 @@ libraries = {
 }
 
 [node name="PlatformStatic" type="StaticBody2D" parent="Platforms"]
-z_index = -1
 position = Vector2(569.935, 105.877)
 rotation = 0.355618
 collision_layer = 16