Fix OIT texture not clearing properly

[lilleyse]

Fixes a bug @likangning93 noticed where OIT textures would not get cleared correctly because of changes in #8205. No CHANGES.md entry required since this change has not been shipped yet.

Previously GlobeDepth would recreate its textures when clearGlobeDepth changed in order to create the primitive framebuffer. This also recreated the main framebuffer, which was fine, but OIT did not have the proper checks to notice that the framebuffer had changed. I considered adding a clearGlodeDepth check in OIT but decided to just separate primitive framebuffer creation and main frambuffer creation in GlobeDepth.

Sandcastle with GeoJSON from http://data.phl.opendata.arcgis.com/datasets/f43e5f92d34e41249e7a11f269792d11_0.geojson. Select CWT from the base layer dropdown to see the bug.

[cesium-concierge]

Thanks for the pull request @lilleyse!

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

[likangning93]

Thanks @lilleyse, works as advertised!