Fix textureSampleBias

The default gradient mult used [1,1,1] and then set one
of them the value that would generate the required mipLevel.
But that was ignoring the that if the required mipLevel is
less then 0 the 1s left over would generate a mipLevel of 0.

For now, make the default [0,0,0]. Another alternative
would be to set them all to the desired value that generates
the desired mipLevel or, we could set them to something less
than the value that generates the desired mip level.

src/webgpu/shader/execution/expression/call/builtin/texture_utils.ts

@@ -1742,7 +1742,9 @@ function softwareTextureReadGrad<T extends Dimensionality>(
   const bias = call.bias === undefined ? 0 : clamp(call.bias, { min: -16.0, max: 15.99 });
   if (call.ddx) {
     const mipLevel = computeMipLevelFromGradientsForCall(call, texture.descriptor.size);
-    const weightMipLevel = mapSoftwareMipLevelToGPUMipLevel(t, stage, mipLevel + bias);
+    const mipLevelCount = texture.descriptor.mipLevelCount ?? 1;
+    const clampedMipLevel = clamp(mipLevel + bias, { min: 0, max: mipLevelCount - 1 });
+    const weightMipLevel = mapSoftwareMipLevelToGPUMipLevel(t, stage, clampedMipLevel);
     return softwareTextureReadLevel(t, stage, call, texture, sampler, weightMipLevel);
   } else {
     return softwareTextureReadLevel(t, stage, call, texture, sampler, (call.mipLevel ?? 0) + bias);
@@ -3273,7 +3275,7 @@ function generateTextureBuiltinInputsImpl<T extends Dimensionality>(
     // choose a derivative value that will select a mipLevel.
     const makeDerivativeMult = (coords: T, mipLevel: number): T => {
       // Make an identity vec (all 1s).
-      const mult = new Array(coords.length).fill(1);
+      const mult = new Array(coords.length).fill(0);
       // choose one axis to set
       const ndx = makeRangeValue({ num: coords.length - 1, type: 'u32' }, i, 8);
       assert(ndx < coords.length);
@@ -3806,7 +3808,7 @@ export function generateSamplePointsCube(
     // choose a derivative value that will select a mipLevel.
     const makeDerivativeMult = (coords: vec3, mipLevel: number): vec3 => {
       // Make an identity vec (all 1s).
-      const mult = new Array(coords.length).fill(1);
+      const mult = new Array(coords.length).fill(0);
       // choose one axis to set
       const ndx = makeRangeValue({ num: coords.length - 1, type: 'u32' }, i, 8);
       assert(ndx < coords.length);