Add basic memory statistics in ModelExperimental

[ptrgags]

This PR partially addresses #9886 by adding memory statistics to the various pipeline stages. Unfortunately, due to the global ResourceCache, it's hard to tell if memory has been shared across multiple models. Some things are likely double-counted in this case. I have some ideas for creating a separate global statistics, but that'll be in a separate PR.

Along the way I tweaked GltfIndexBufferLoader to load the original indices as a Buffer in addition to the typed array. This way it doesn't need to be re-uploaded every time the ModelExperimental pipeline is re-run. It also simplifies some of of the code in buildDrawCommands().

To Do:

• Sweep through the diff and see what other files like StructuralMetadata need unit tests.
• In the test Cesium3DTileset > verify memory usage statistics the batch table statistics seems to have the right value, but the line in Cesium3DTilesetStatistics that increments the total for the tileset doesn't seem to be run until later. Not sure why that is, I haven't changed the tileset statistic code much.
• There's a test in BatchTableHierarchy that works when run in isolation but not when run with other tests in the same suite... sounds like some state is being modified between tests, but I haven't yet found the problem.
• In the 3D Tiles Inspector, the selected features/points/triangles are all returning 0, not sure why that is.
• Sync with main and fix merge conflicts.
• Update Add statistics to ModelExperimental, ModelExperimental3DTileContent #9886 to include details about the ResourceCache.statistics idea.

[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.

[ptrgags]

Did a pass through the code to make a checklist of files to add unit tests for. Got a couple of possible leads for unit test debugging along the way.

[ptrgags]

@j9liu updated!

[j9liu]

@ptrgags did another sweep -- just a few cases where the stage assumes things exist, when in reality they're not required by the schema.

[j9liu]

Should we test that _countMaterialTextures doesn't update for a model without material textures? (e.g. simple texture model, or simple model without a material?)

Also I don't think getAllTextureReaders accounts for if a model doesn't have a material

[ptrgags]

I tried this with Triangle.gltf and discovered that it still has a default material! Though surprisingly, it wasn't any GltfLoader code, it was https://github.com/CesiumGS/gltf-pipeline this time! I left some comments explaining this, because it was non-obvious

[j9liu]

@ptrgags also, does this need an entry in CHANGES.md?

[ptrgags]

@j9liu updated!

[j9liu]

@ptrgags I noticed a typo in CHANGES.md and a leftover console.log, so I just pushed those changes. Will merge when Travis passes!