Fix sRGB luminance texture conversion

[issam3105]

Objective

Fix a regression introduced by 0539e4daf.

That commit started preserving grayscale images as R8Unorm. For sRGB textures, this loses sRGB decoding and sampling the texture as RGB returns the luminance only in the red channel.

Solution

Expand Luma8 and LumaA8 images to RGBA when they are loaded as sRGB

Showcase

CompareVolume and CompareTransmission use a grayscale JPEG as an emissive texture. TransmissionOrderTest uses a grayscale PNG as a base color texture. In both cases, grayscale areas were rendered in red instead of grayscale.

Testing

• Reviewers can render CompareVolume.glb, CompareTransmission.glb, and TransmissionOrderTest.glb and check that their grayscale textures are no longer red.