Race condition when loading local tileset with custom materials

[lilleyse]

When loading a tileset with custom materials the materialHasCesiumNodes function doesn't return the correct result if the USD material hasn't been populated into Fabric yet. I only ever notice this when loading tilesets on disk. The end result is usually a blank material.

To fix this we should update materialHasCesiumNodes to look at the USD stage rather than the Fabric stage, if possible.

I can work around it by adding a 1 second wait to each HTTP request. Obviously this is a big hack, it just helps unblock other things I'm working on.

using namespace std::this_thread;
using namespace std::chrono_literals;
using std::chrono::system_clock;
sleep_for(1s);

Test data: feature-id-attribute-mdl.zip (make sure to update paths to the tileset.json in the .usda for your system)