colorLib_test: test that we accept either tuple or list for COLRv0 layer

https://github.com/googlefonts/ufo2ft/issues/426

Tests/colorLib/builder_test.py

@@ -27,6 +27,37 @@ def test_buildCOLR_v0():
     assert colr.ColorLayers["b"][1].colorID == 0
 
 
+def test_buildCOLR_v0_layer_as_list():
+    # when COLRv0 layers are encoded as plist in UFO lib, both python tuples and
+    # lists are encoded as plist array elements; but the latter are always decoded
+    # as python lists, thus after roundtripping a plist tuples become lists.
+    # Before FontTools 4.17.0 we were treating tuples and lists as equivalent;
+    # with 4.17.0, a paint of type list is used to identify a PaintColrLayers.
+    # This broke backward compatibility as ufo2ft is simply passing through the
+    # color layers as read from the UFO lib plist, and as such the latter use lists
+    # instead of tuples for COLRv0 layers (layerGlyph, paletteIndex) combo.
+    # We restore backward compat by accepting either tuples or lists (of length 2
+    # and only containing a str and an int) as individual top-level layers.
+    # https://github.com/googlefonts/ufo2ft/issues/426
+    color_layer_lists = {
+        "a": [["a.color0", 0], ["a.color1", 1]],
+        "b": [["b.color1", 1], ["b.color0", 0]],
+    }
+
+    colr = builder.buildCOLR(color_layer_lists)
+
+    assert colr.tableTag == "COLR"
+    assert colr.version == 0
+    assert colr.ColorLayers["a"][0].name == "a.color0"
+    assert colr.ColorLayers["a"][0].colorID == 0
+    assert colr.ColorLayers["a"][1].name == "a.color1"
+    assert colr.ColorLayers["a"][1].colorID == 1
+    assert colr.ColorLayers["b"][0].name == "b.color1"
+    assert colr.ColorLayers["b"][0].colorID == 1
+    assert colr.ColorLayers["b"][1].name == "b.color0"
+    assert colr.ColorLayers["b"][1].colorID == 0
+
+
 def test_buildCPAL_v0():
     palettes = [
         [(0.68, 0.20, 0.32, 1.0), (0.45, 0.68, 0.21, 1.0)],