Minimum required attribution for Google 3D Tiles

[ggetz]

By default, createGooglePhotorealistic3DTileset no longer shows credits on screen, as this is compliant with the minimum required attribution. Traditionally CesiumJS uses the minimum necessary to be compliant as to avoid any confusion about what is required.

To restore this behavior, pass the option showCreditsOnScreen: true.

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ 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.

[jjhembd]

Looks good, thanks @ggetz!