Mod: Show regressions in shape colouring

This PR demonstrates the known regressions of the recently merged material branch:
* Changing the transparency after setting color per face will reset them
* The result of boolean operations or compound doesn't inherit the colour of its input objects
* If colour is set per face to a boolean operaton object then saving and restoring the file causes weird rendering behaviour
  because material binding is set to PER_PART but only a single colour is defined
* If a shape inside a part container has set colour per face then saving and restoring as STEP file causes weird rendering
behaviour for the same reason
* Shape binder or datum objects don't show the correct default shape colour

src/Mod/Import/CMakeLists.txt

@@ -47,6 +47,7 @@ set(Import_Scripts
 
 if(BUILD_GUI)
     list (APPEND Import_Scripts InitGui.py)
+    list (APPEND Import_Scripts TestImportGui.py)
 endif(BUILD_GUI)
 
 add_custom_target(ImportScripts ALL

src/Mod/Import/InitGui.py

@@ -35,6 +35,8 @@ FreeCAD.changeImportModule("STEP with colors (*.step *.STEP *.stp *.STP)", "Impo
 FreeCAD.changeExportModule("STEP with colors (*.step *.stp)", "Import", "ImportGui")
 FreeCAD.changeExportModule("glTF (*.gltf *.glb)", "Import", "ImportGui")
 
+App.__unit_test__ += ["TestImportGui"]
+
 """
 class ImportWorkbench ( Workbench ):
     "Import workbench object"

src/Mod/Import/TestImportGui.py

@@ -0,0 +1,91 @@
+# **************************************************************************
+#   Copyright (c) 2024 Werner Mayer <wmayer[at]users.sourceforge.net>     *
+#                                                                         *
+#   This file is part of FreeCAD.                                         *
+#                                                                         *
+#   FreeCAD is free software: you can redistribute it and/or modify it    *
+#   under the terms of the GNU Lesser General Public License as           *
+#   published by the Free Software Foundation, either version 2.1 of the  *
+#   License, or (at your option) any later version.                       *
+#                                                                         *
+#   FreeCAD is distributed in the hope that it will be useful, but        *
+#   WITHOUT ANY WARRANTY; without even the implied warranty of            *
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU      *
+#   Lesser General Public License for more details.                       *
+#                                                                         *
+#   You should have received a copy of the GNU Lesser General Public      *
+#   License along with FreeCAD. If not, see                               *
+#   <https://www.gnu.org/licenses/>.                                      *
+#                                                                         *
+# **************************************************************************
+
+import os
+import tempfile
+import unittest
+import FreeCAD as App
+import ImportGui
+from pivy import coin
+
+
+class ExportImportTest(unittest.TestCase):
+    def setUp(self):
+        TempPath = tempfile.gettempdir()
+        self.fileName = TempPath + os.sep + "ColorPerFaceTest.step"
+        self.doc = App.newDocument()
+
+    def tearDown(self):
+        App.closeDocument(self.doc.Name)
+
+    def testSaveLoadStepFile(self):
+        """
+        Create a STEP file with color per face
+        """
+        part = self.doc.addObject("App::Part", "Part")
+        box = part.newObject("Part::Box", "Box")
+        self.doc.recompute()
+
+        box.ViewObject.DiffuseColor = [
+            (1.0, 0.0, 0.0, 0.0),
+            (1.0, 0.0, 0.0, 0.0),
+            (1.0, 0.0, 0.0, 0.0),
+            (1.0, 0.0, 0.0, 0.0),
+            (1.0, 1.0, 0.0, 0.0),
+            (1.0, 1.0, 0.0, 0.0),
+        ]
+
+        ImportGui.export([part], self.fileName)
+
+        self.doc.clearDocument()
+        ImportGui.insert(name=self.fileName, docName=self.doc.Name, useLinkGroup=True)
+
+        part_features = list(filter(lambda x: x.isDerivedFrom("Part::Feature"), self.doc.Objects))
+        self.assertEqual(len(part_features), 1)
+        feature = part_features[0]
+
+        self.assertEqual(len(feature.ViewObject.DiffuseColor), 6)
+        self.assertEqual(feature.ViewObject.DiffuseColor[0], (1.0, 0.0, 0.0, 0.0))
+        self.assertEqual(feature.ViewObject.DiffuseColor[1], (1.0, 0.0, 0.0, 0.0))
+        self.assertEqual(feature.ViewObject.DiffuseColor[2], (1.0, 0.0, 0.0, 0.0))
+        self.assertEqual(feature.ViewObject.DiffuseColor[3], (1.0, 0.0, 0.0, 0.0))
+        self.assertEqual(feature.ViewObject.DiffuseColor[4], (1.0, 1.0, 0.0, 0.0))
+        self.assertEqual(feature.ViewObject.DiffuseColor[5], (1.0, 1.0, 0.0, 0.0))
+
+        sa = coin.SoSearchAction()
+        sa.setType(coin.SoMaterialBinding.getClassTypeId())
+        # We need an easier way to access nodes of a display mode
+        sa.setInterest(coin.SoSearchAction.ALL)
+        sa.apply(feature.ViewObject.RootNode)
+        paths = sa.getPaths()
+
+        bind = paths.get(2).getTail()
+        self.assertEqual(bind.value.getValue(), bind.PER_PART)
+
+        sa = coin.SoSearchAction()
+        sa.setType(coin.SoMaterial.getClassTypeId())
+        # We need an easier way to access nodes of a display mode
+        sa.setInterest(coin.SoSearchAction.ALL)
+        sa.apply(feature.ViewObject.RootNode)
+        paths = sa.getPaths()
+
+        mat = paths.get(2).getTail()
+        self.assertEqual(mat.diffuseColor.getNum(), 6)