Clean up webgpu_compute_cloth example

[WestLangley]

This PR changes the material color so the sheen is readily apparent.

[WestLangley]

There is a design problem that will have to be addressed in another PR.

The material color set in the GUI does not match the rendered material color.

[Mugen87]

Instead of:

materialFolder.addColor( clothMaterial, 'color' );

try it with:

materialFolder.addColor( { color: clothMaterial.color.getHex( THREE.SRGBColorSpace ) }, 'color' ).onChange( value => clothMaterial.color.set( value ) );

Same for the sheen color.

[WestLangley]

try it with:

That results in rounding problems in my testing.

Compare with the approach used by the matcap example.