Turn off throttle option when requesting for an availability tile from the parent terrain layer

[baothientran]

The function checkLayer() in the CesiumTerrainProvider.js file is responsible for requesting an availability tile for the parent layer. However, the promise is delayed for a long time or never get resolved especially where the tile is overlapped with the child layer, which makes QuadtreePrimitive upsamples the tile from the parent tile. Turning off the throttle option in the Request object does solves the issue though. This is consistent with other requests coming from the other terrain code

This is the parent layer when all the tiles are available. Notice the three lines on the head of the two tiles at level 10

This is the missing tiles from the parent layer when delay happens. Notice there are no three lines on the head of the tiles at level 10. Keeping zooming in doesn't solve the problem

@lilleyse Can you please take a look at it? Please let me know if I need to add anything

[cesium-concierge]

Thanks for the pull request @baothientran!

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[lilleyse]

@baothientran Definitely 👍 on the change. The terrain and imagery system used to throttle requests in the RequestScheduler but it clashed with 3D Tiles' priority system so we reverted it and let terrain prioritize/throttle according to its own rules. I think this code just never got reverted.

Can you make the same change for other terrain/imagery providers? A search for throttle: true brings up a few more results. Ultimately throttle: true should be removed everywhere except in Cesium3DTile, ResourceSpec, and RequestSchedulerSpec.

[baothientran]

@lilleyse I turned off the throttle options for all the terrain and imagery provider. Some tests are fixed along the way, but it shouldn't change the testing cases' purposes

[lilleyse]

@baothientran could you add the fix to CHANGES.md?

[lilleyse]

@kring can you confirm that this is a good fix?

[kring]

Isn't it a problem that we're no longer calling createRequest here? So the request has no URL, for one thing.

[baothientran]

it seems like createRequest() returns an empty request object with only throttleByServer option on. I kind of create the new Request for only the test case. Should I modify the createRequest() with throttle to be true for all of them?

[kring]

Oh ok, I hadn't realized createRequest also created a request without a URL. That makes sense, no need to change anything.