Incorrect thumbnail gamma with KTX

[echadwick-artist]

1. Download a GLB which contains KTX2 assets inside it, for example https://github.com/KhronosGroup/3D-Formats-Guidelines/blob/main/samples/StainedGlassLamp/StainedGlassLamp-KTX-BasisU-Volume-ETC1S.glb
2. Go to https://phasmatic3d.github.io/glTF-Sample-Viewer-KTX-demo/
3. Drag and drop the GLB into the 3d view.
4. Go to the Texture Compressor tab.
5. Open a new Chrome window, and repeat the above except using a GLB with PNG/JPG textures, for example https://github.com/KhronosGroup/3D-Formats-Guidelines/blob/main/samples/StainedGlassLamp/StainedGlassLamp-JPG-PNG-Volume.glb
6. Compare the two windows side-by-side.

Result: some thumbnails are displayed with incorrect gamma.

• Normal/occlusion/rough/metal textures are overbright (they should be displayed in linear colorspace).
• Basecolor and emissive seem to be OK.
• KTX2 with alpha channel seems to be displayed with black, compared to gray for a PNG with alpha.

Screenshots comparing glTF with KTX vs glTF with PNG/JPG:

[abasilak]

The commits https://github.com/phasmatic3d/glTF-Sample-Viewer-KTX/commit/994ec01d476c07523fdb81bced997af750a171b7 & https://github.com/phasmatic3d/glTF-Sample-Viewer-KTX/commit/0212e0c28409c72570f13c72413852ef2953012f solved this issue.

[echadwick-artist]

Looks great, thank you so much!