fwidth will throw an error in WebGPU renderer

[xiaoiver]

SDF in Lesson9 will throw the following error:

Error while parsing WGSL: :116:16 error: 'fwidth' must only be called from uniform control flow
    let _e16 = fwidth(_e15);

[xiaoiver]

Turn off diagnostic for now in WGSL.

const diagnosticDerivativeUniformityHeader =
      this.device.queryVendorInfo().platformString === 'WebGPU'
        ? 'diagnostic(off,derivative_uniformity);'
        : '';