Cesium not loading higher level tiles when base image provider hits limit

Hi,

I am having trouble rendering tiles in other layers once the main image provider doesn't have tiles of the same level. For example, I am using the default Bing image provider. I have two drone surveys that I have broken into tiles, supporting zoom levels 17-22, which I am loading as UrlTemplateImageryProviders. One survey is in a location where the Bing has tiles up to level 22, and the survey loads fine. The other survey is from a more remote location, where Bing only has tiles of zoom 14 or larger. It seems that Cesium doesn't attempt to load at a level greater than 14, no matter how close I move the camera to ground level.

It would be nice if Cesium could support loading tiles from other image layers, even if the base layer has reached its limit.

Reproduce the issue with the default Bing image provider and two UrlTemplateImageryProviders covering zoom levels 17–22, comparing a location where Bing reaches level 22 with one where it stops at level 14. Trace how imagery layers choose levels when the base provider has no higher tiles. Done means the other imagery layers continue loading at higher levels near the ground despite the base layer limit.

Written by the indexing model from the issue text.