Bug 264371

textureSampleGrad should map to sample which takes a gradient (page 202 of the Metal shading language spec)

Open https://webgpu.github.io/webgpu-samples/samples/normalMap.

Note the failure:

MSL compilation error: Error Domain=MTLLibraryErrorDomain Code=3 "program_source:120:26: error: use of undeclared identifier 'textureSampleGrad'
        if ((local7.z >= textureSampleGrad(global11, global8, local7.xy, local2, local3).r)) {
                         ^
" UserInfo={NSLocalizedDescription=program_source:120:26: error: use of undeclared identifier 'textureSampleGrad'
        if ((local7.z >= textureSampleGrad(global11, global8, local7.xy, local2, local3).r)) {


it comes from this WGSL:

 // Walk the depth texture, and stop when the ray intersects the depth map
  var pos = vec3(startUV, 0);
  for (var i = 0; i < 32; i++) {
    if (pos.z >= textureSampleGrad(depthTexture, textureSampler, pos.xy, ddx, ddy).r) {
      break; // Hit the surface
    }
    pos += posDelta;
  }

in https://webgpu.github.io/webgpu-samples/samples/normalMap#./normalMap.wgsl