Simplify `layered-textures` example

Most of the members of `StandardMaterial` used on the `ExtendedMaterial` are unused, so replace the `ExtendedMaterial` that only contains the texture arrays and create the `PbrInput` type from scratch without needing to vendor `pbr_fragment.wgsl` from Bevy, also set up bindless materials, which is trivial if you only have textures