Materials lost when transforming a SnapshotDb

[MichaelBelousov]

Copied from iTwin/itwinjs-core#5307, original issue posted by @fatalstar

Describe the bug
I try to transform a SnapshotDb to another, but materials lost.
And then, I try to transform it to an empty SnapshotDb，materials get lost and error.

To Reproduce
Steps to reproduce the behavior:

1. Convert House_Model.dgn to a local house.bim file. And do same thing with testY1.dgn.
2. Build an iModelTransformer with core-transformer@3.4.4
3. Excute iModelTransformer.processAll( house.bim, testY1.bim, no options),
4. When transform done, excute targetDb.saveChanges( )
5. Open the targetDb in view, and could see the materials lost.

Expected behavior
Materials exist and be correct.

Screenshots
The house.bim view :

The result.bim view: (materials get lost)

If transform to an empty db, and view: (roof material lost, wall and steps meterial error)

Desktop (please complete the applicable information):

• iTwin.js Version [3.4.4]
• iTwin.js Version [3.6.3] also tested

[MichaelBelousov]

I have confirmed that this is because several json properties are not tracked by the transformer. I have added a tool to the test-app to detect them, ran it on the house_model, and am now able to find several. Can use it as a starting point to add better support for tracking known json properties in the transformer.

[MichaelBelousov]

@fatalstar I wanna apologize for how long this is taking. I had to shift focus to general transformation performance issues. I did work on this, but am yet to complete it. If you want me to give you a special monkey patch for iTwin.js to fix this, let me know. I will get back to this, I am just considering the other work higher priority.