Add tests for the primitive_index builtin

[toji]

Adds some initial tests for the primitive_id WGSL builtin. Currently uses enable chromium_experimental_primitive_id to enable, but that can change once the spec is fleshed out and accepted by the group. Feature is not exposed to compat mode.

Associated spec change is gpuweb/gpuweb#5273 I'm totally fine if we want to wait to land this till that is approved by the group and this can revert to using the final feature strings.

Issue: gpuweb/gpuweb#1786

---

Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[greggman]

The code looks good to me as a start but maybe some todos?

• It's only testing that (num primitives rendered - 1) is written to the texture. It is not checking that primitive_id is ever anything other than (num primitives rendered - 1). In other words, if triCount = 5 and primitive_id was always 4 it would pass the test. Where as it's supposed to return 0 for primitive 0, 1 for primitive 1, etc....

• It's not checking other topologies (point-list, line-list, line-strip, triangle-list)

[greggman]

Do we need to check behavior for these cases?

If your 2d vertex data is

0.1, 0.1    0.2, 0.1    0.1, 0.2   // normal triangle
0.3, 0.3    0.3, 0.3    0.3, 0.3   // zero size triangle
2.1, 2.1    2.2, 2.1    2.1, 2.2   // normal triangle but off screen
0.00001, 0.00001    0.00002, 0.00001   0.00001, 0.00002   // triangle whose area covers no pixels.
0.1, 0.1    0.1, 0.2   0.2, 0.1     // backface culling on, a culled triangle.

I believe all of these are supposed to be counted.

[toji]

That's an excellent point! I had meant to add cases for culled or degenerate primitives but it slipped my mind when submitting. I'll add some.

(Been working on a couple of other related tasks in other areas, like the spec, which is why I haven't updated this yet. Will loop back around to it soon.)

[toji]

Please take another look. Coves more cases now, including all topology types, testing multiple primitive indices in a single draw, and degenerate/offscreen/reset primitives.

[greggman]

lgtm - dang I wish there was some diagrams. I had to manually graph everything 😅 - we should write a vscode cts extension where you can add like // CTS: vec2f and it reads the array below the comment and draws an SVG graph 😛

      //           0,2
      //            +
      //           /|
      //          /.|
      //         +--+--+
      //        /|..|  |
      //       /.|..|  |
      // -2,0 +--+--O--+--+ 2,0
      //         |  |  |
      //         |  |  |
      //         +--+--+
      //            |
      //            |
      //            +
      //           0,-2
       0, 0,
      -2, 0,
       0, 2,

      //           0,2
      //            +
      //            |\
      //            |.\
      //         +--+--+
      //         |  |..|\
      //         |  |..|.\
      // -2,0 +--+--O--+--+ 2,0
      //         |  |  |
      //         |  |  |
      //         +--+--+
      //            |
      //            |
      //            +
      //           0,-2
       0, 0,
       0, 2,
       2, 0,
       
      //           0,2
      //            +
      //            |
      //            |
      //         +--+--+
      //         |  |  |
      //         |  |  |
      // -2,0 +--+--O--+--+ 2,0
      //       \.|..|  |
      //        \|..|  |
      //         +--+--+
      //          \.|
      //           \|
      //            +
      //           0,-2
       0, 0,
      -2, 0,
       0,-2,
       
      //           0,2
      //            +
      //            |
      //            |
      //         +--+--+
      //         |  |  |
      //         |  |  |
      // -2,0 +--+--O--+--+ 2,0
      //         |  |..|./
      //         |  |..|/
      //         +--+--+
      //            |./
      //            |/
      //            +
      //           0,-2
       0, 0,
       0,-2,
       2, 0,


      //         v2
      //          +
      //          |
      //          |
      //       +--+--+
      //       |  |  |
      //       |v1|v4|
      // v0 +--+--.--+--+ v3
      // v7    |  |  |
      //       |  |  |
      //       +--+--+
      //          |
      //          |
      //          +
      //          v5,v6
      //
      //  #0  #1  #2  #3  #4  #5
      //   +  +   +   +-+ +   +-+
      //  /|  |\  |\  |/  |    \|
      // +-+  +-+ +-+ +   +     +
-2, 0, 0, 0, 0, 2, 2, 0, 0, 0, 0, -2, 0, 0, -2, 0

      //         v1,v5   v2,v6
      //       +---*---+---*---+
      //       |       |       |
      //       |       |       |
      //       |       |       |
      //       +-------o-------+
      //       |       |       |
      //       |       |       |
      //       |       |       |
      //       +---*---+---*---+
      //         v0,v4   v3,v7
-0.5, -1, -0.5, 1, 0.5, 1, 0.5, -1, -0.5, -1, -0.5, 1, 0.5, 1, 0.5, -1

      //         v1,v5   v2,v6
      //       +-------+-------+
      //       |       |       |
      //       |   *   |   *   |
      //       |       |       |
      //       +-------o-------+
      //       |       |       |
      //       |   *   |   *   |
      //       |       |       |
      //       +-------+-------+
      //         v0,v4   v3,v7

[toji]

Added your awesome ASCII art to the PR when rebasing. Thanks!