Fix per-feature post-processing with ModelExperimental

[ptrgags]

the Sandcastles "Per-Feature Post Processing" and "Custom Per-Feature Post Processing" were not working correctly when ModelExperimental was used.

The fix was simple, there was a missing private getter model.pickIds.

• ModelExperimental custom per-feature post processing
• ModelExperimental per-feature post processing

@j9liu could you review?

[cesium-concierge]

Thanks for the pull request @ptrgags!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[j9liu]

@ptrgags looks good, I confirmed this works with the Custom Per-Feature Post Process and Per-Feature Post Processing sandcastles!

Just waiting on Travis to pass, then I'll merge