metallicRoughnessTexture encoding is wrong

The spec [says](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#metallic-roughness-material):

> The textures for metalness and roughness properties are packed together in a single texture called metallicRoughnessTexture. Its green channel contains roughness values and its blue channel contains metalness values. This texture *MUST* be encoded with linear transfer function and *MAY* use more than 8 bits per channel.

But metallic-roughness textures in Sponza folder are encoded with sRGB. Checked it with ImageMagick and PVRTexTool.

Inspect the metallic-roughness textures in the Sponza folder and compare their transfer function with the glTF 2.0 metallicRoughnessTexture requirement, using ImageMagick or PVRTexTool as in the report. Done means the affected textures use linear encoding while preserving the specified green roughness and blue metalness channels.

Written by the indexing model from the issue text.