Cleanup: No longer require VSE Strip struct memory layout to never change

Previously code that was reading Strip data assumed that seqbasep
and channels members would stay at fixed offsets within a struct,
forever into the future. Fix this by inferring their offsets from
the file SDNA data where needed.

Actual Strip DNA layout is not changed in this commit yet.

Co-authored-by: Sergey Sharybin <sergey@blender.org>
Pull Request: https://projects.blender.org/blender/blender/pulls/142940

tests/python/CMakeLists.txt

@@ -1297,6 +1297,12 @@ if(TEST_SRC_DIR_EXISTS)
     --
     --testdir "${TEST_SRC_DIR}/sequence_editing"
   )
+
+  add_blender_test(
+    sequencer_load_meta_stack
+    ${TEST_SRC_DIR}/sequence_editing/vse_load_meta_stack.blend
+    --python ${TEST_PYTHON_DIR}/sequencer_load_meta_stack.py
+  )
 endif()
 
 # ------------------------------------------------------------------------------

tests/python/sequencer_load_meta_stack.py

@@ -0,0 +1,46 @@
+# SPDX-FileCopyrightText: 2020-2022 Blender Authors
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# To run all tests, use
+# BLENDER_VERBOSE=1 ./bin/blender --background ../tests/files/sequence_editing/vse_load_meta_stack.blend --python ../blender/tests/python/sequencer_load_meta_stack.py
+# (that assumes the test is run from a build directory in the same directory as the source code)
+import bpy
+
+import argparse
+import sys
+import unittest
+
+
+class SequencerLoadMetastaskTest(unittest.TestCase):
+    def get_sequence_editor(self):
+        return bpy.context.scene.sequence_editor
+
+    def test_meta_stack(self):
+        sequence_editor = self.get_sequence_editor()
+
+        meta_stack = sequence_editor.meta_stack
+        self.assertEqual(len(meta_stack), 1)
+        self.assertEqual(meta_stack[0].name, "MetaStrip")
+
+        self.assertEqual(len(meta_stack[0].sequences), 1)
+        self.assertEqual(meta_stack[0].sequences[0].name, "Color")
+
+        # accesses ed->seqbasep through screen_ctx_selected_editable_sequences
+        bpy.context.copy()
+
+
+def main():
+    argv = [sys.argv[0]]
+    if '--' in sys.argv:
+        argv += sys.argv[sys.argv.index('--') + 1:]
+
+    parser = argparse.ArgumentParser()
+
+    args, remaining = parser.parse_known_args(argv)
+
+    unittest.main(argv=remaining)
+
+
+if __name__ == "__main__":
+    main()