Fix: Grease Pencil: Wrong layer attributes after reorder

Reordering layer nodes by drag-drop, move up/down, add new etc. swaps
layer attributes with wrong layers. This is due to mistake in map
`new_by_old_map`. Values of this map is used as indices in src array.
And keys are indices of dst array. Expected behavior is to copy attribute from
old position (`layer_i_old`) of src array to new position (`layer_i_new`) of
dst array. See: `array_utils::gather`.

Noticed during !141772.

Pull Request: https://projects.blender.org/blender/blender/pulls/141935

tests/python/bl_pyapi_grease_pencil.py

@@ -24,12 +24,17 @@ class TestGreasePencil(unittest.TestCase):
 
 
 class TestGreasePencilLayers(unittest.TestCase):
+    tint_factors = [0.3, 0.6, 0.9]
+
     def setUp(self):
         self.gp = bpy.data.grease_pencils_v3.new("test_grease_pencil")
         self.gp.layers.new("test_layer01")
         self.gp.layers.new("test_layer02")
         self.gp.layers.new("test_layer03")
 
+        for i, layer in enumerate(self.gp.layers):
+            layer.tint_factor = self.tint_factors[i]
+
     def tearDown(self):
         bpy.data.grease_pencils_v3.remove(self.gp)
         del self.gp
@@ -69,6 +74,14 @@ class TestGreasePencilLayers(unittest.TestCase):
         self.assertEqual(self.gp.layers[1].name, "test_layer01")
         self.assertEqual(self.gp.layers[2].name, "test_layer03")
 
+    def test_grease_pencil_layers_attribute_reorder(self):
+        layer = self.gp.layers[0]
+        self.gp.layers.move_top(layer)
+        # Check layer attribute
+        self.assertEqual(round(self.gp.layers[0].tint_factor, 1), self.tint_factors[1])
+        self.assertEqual(round(self.gp.layers[1].tint_factor, 1), self.tint_factors[2])
+        self.assertEqual(round(self.gp.layers[2].tint_factor, 1), self.tint_factors[0])
+
 
 class TestGreasePencilFrame(unittest.TestCase):
     def setUp(self):