Adds UV Channel Selection Support for Base Colour, Metallic-Roughness, Normal Map and Occlusion Textures

[HulloImJay]

I'm a Rust and Bevy newbie and this is my first PR, so I expect it not to pass muster! However, I would appreciate any feedback ;)

Objective

• I wanted to be able to use the second UV channel when sampling textures.

Solution

• Adds channel-selection properties to the Standard Material.
• These properties are used in the fragment shader if a second set of UVs exists on the mesh.
• Applies to base colour, metallic-roughness, normal map, and occlusion.
• Adds one example demonstrating the feature.

The new example I tested the changes with: Spheres on the left use UV0 and those on the right use UV1 (which is just a scaled copy — sufficient to see a difference). The top row tests base colour and metallic-roughness, and the bottom tests normal map and occlusion.

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Changelog

• Changes across bevy_pbr to add the new properties and to use them within the fragment shader where appropriate.
• A new example which borrows from the 3D shapes but demonstrates using either the first or second UV channels for various textures.

Migration Guide

• I attempted to make these changes s.t. existing features would not change...

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The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

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The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.