Use LEQUAL instead of LESS for depth testing models.

[kring]

This means that a glTF that has a primitive with filled faces followed by a primitive with edge lines will look like this:

Where previously it looked like this:

If this looks ok to everyone it would be really great to get it into the March CesiumJS release!

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ 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.
• Works (or fails gracefully) in IE11.