Fixed issue where we constantly fail to load the same image.

[tfili]

Fixes #6567

Can easily be tested with the following. Before this fix it tries to load etna.jpg hundreds of times.

var viewer = new Cesium.Viewer('cesiumContainer');
var options = {
    camera : viewer.scene.camera,
    canvas : viewer.scene.canvas
};

viewer.dataSources.add(Cesium.KmlDataSource.load('https://gist.githubusercontent.com/shunter/03d24977213616ae9992f443d3c051ea/raw/6d2aeffc8e667b9100a2875388d19e9c64b945fd/KML_Samples.kml', options));

[cesium-concierge]

@tfili, thanks for the pull request! Maintainers, we have a signed CLA from @tfili, so you can review this at any time.

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[hpinkos]

@tfili is there a good way write a unit test to make sure the material only tries to load the resource once?

Other than that this change looks fine to me

[tfili]

@tfili is there a good way write a unit test to make sure the material only tries to load the resource once?

It doesn't look like it. It'll be pretty difficult and a ton of mocking to test this. I don't think it's worth the time it'll take.

[hpinkos]

Thanks @tfili