Add debug info to fragment builtins.

[greggman]

This lets you visualize the expected results so it's easy to see what's being tested.

for example: https://gpuweb.github.io/cts/standalone/?debug=1&runnow=1&q=webgpu:shader,execution,shader_io,fragment_builtins:inputs,interStage:nearFar=[0,1];sampleCount=4;interpolation={%22type%22:%22perspective%22,%22sampling%22:%22sample%22}

DEBUG: 
    +-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+
s0: | 3.905 4.905 5.905 6.905 | 3.010 4.010 5.010 6.010 | 2.197 3.197 4.197 5.197 | 1.454 2.454 3.454 4.454 |
s1: | 3.557 4.557 5.557 6.557 | 2.679 3.679 4.679 5.679 | 1.881 2.881 3.881 4.881 | 1.151 2.151 3.151 4.151 |
s2: | 4.415 5.415 6.415 7.415 | 3.439 4.439 5.439 6.439 | 2.556 3.556 4.556 5.556 | 1.754 2.754 3.754 4.754 |
s3: | 4.045 5.045 6.045 7.045 | 3.087 4.087 5.087 6.087 | 2.221 3.221 4.221 5.221 | 1.434 2.434 3.434 4.434 |
    +-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+
s0: | 4.446 5.446 6.446 7.446 | 3.430 4.430 5.430 6.430 | 2.514 3.514 4.514 5.514 | 1.686 2.686 3.686 4.686 |
s1: | 4.060 5.060 6.060 7.060 | 3.064 4.064 5.064 6.064 | 2.167 3.167 4.167 5.167 | 1.355 2.355 3.355 4.355 |
s2: | 5.057 6.057 7.057 8.057 | 3.935 4.935 5.935 6.935 | 2.932 3.932 4.932 5.932 | 2.031 3.031 4.031 5.031 |
s3: | 4.643 5.643 6.643 7.643 | 3.544 4.544 5.544 6.544 | 2.562 3.562 4.562 5.562 | 1.679 2.679 3.679 4.679 |
    +-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+
s0: | 5.122 6.122 7.122 8.122 | 3.947 4.947 5.947 6.947 | 2.902 3.902 4.902 5.902 | 1.967 2.967 3.967 4.967 |
s1: | 4.688 5.688 6.688 7.688 | 3.539 4.539 5.539 6.539 | 2.516 3.516 4.516 5.516 | 1.601 2.601 3.601 4.601 |
s2: | 5.872 6.872 7.872 8.872 | 4.557 5.557 6.557 7.557 | 3.399 4.399 5.399 6.399 | 2.370 3.370 4.370 5.370 |
s3: | 5.402 6.402 7.402 8.402 | 4.116 5.116 6.116 7.116 | 2.984 3.984 4.984 5.984 | 1.978 2.978 3.978 4.978 |
    +-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+
s0: | 5.990 6.990 7.990 8.990 | 4.602 5.602 6.602 7.602 | 3.386 4.386 5.386 6.386 | 2.313 3.313 4.313 5.313 |
s1: | 5.494 6.494 7.494 8.494 | 4.139 5.139 6.139 7.139 | 2.952 3.952 4.952 5.952 | 1.904 2.904 3.904 4.904 |
s2: | 6.941 7.941 8.941 9.941 | 5.359 6.359 7.359 8.359 | 3.990 4.990 5.990 6.990 | 2.794 3.794 4.794 5.794 |
s3: | 6.397 7.397 8.397 9.397 | 4.854 5.854 6.854 7.854 | 3.520 4.520 5.520 6.520 | 2.353 3.353 4.353 5.353 |
    +-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+-- x --- y --- z --- w --+

[github-actions]

Results for build job (at 33e9bae):

 Test case/subcase counts did not change.

[kainino0x]

great!