Broken texture after etc1s conversion

[hybridherbst]

Describe the bug
The attached model runs through gltf-transform etc1s without error, but the resulting texture is 8x0 pixels and can't be loaded anymore (throws exception in gltf.report and elsewhere).

To Reproduce
Steps to reproduce the behavior:

1. Download TexturedCube.zip
2. Run gltf-transform etc1s TexturedCube.glb TexturedCube.etc1s.glb
3. Open the result in gltf.report

Expected behavior
Has same appearance as original file, does not throw exception.
Alternative: gltf-transform should log an error if this file can't be processed for some reason (but generally I think "everything should pass" in the sense of a pipeline tool/step).

Additional context
The error logged is

Error: THREE.KTX2Loader:	Invalid texture
    at a816abbe-5888-4dab-9200-d3bd76c69d3a:29:1367
    at a816abbe-5888-4dab-9200-d3bd76c69d3a:29:1887
(anonymous) @ a816abbe-5888-4dab-9200-d3bd76c69d3a:29
bundle.js:2 THREE.GLTFLoader: Couldn't load texture blob:https://gltf.report/7406afd6-8a1b-48ad-88ed-fba2ea534bf9

[donmccurdy]

Likely same root cause as #503 (comment) – thanks!

[donmccurdy]

To be fixed in #504.