WebGL2: gl.getAttribLocation for built-in attribute 'gl_VertexID' returns incorrect value (should be -1, got 0)

[EndlessJour9527]

Bug Description

Running Khronos/WebGL2 conformance test (active-built-in-attribs.html) in JSAR Runtime:

shouldBe('activeInfo.name', '"gl_VertexID"');
attribLoc = gl.getAttribLocation(prog, 'gl_VertexID');
wtu.glErrorShouldBe(gl, gl.NO_ERROR, "Should be able to request the location of a built-in.");
shouldBe('attribLoc', '-1');

Test fails: attribLoc should be -1. Was 0.

Spec Reference

• WebGL 2.0 Spec §5.18 gl.getAttribLocation
  • For built-in attributes (like gl_VertexID, gl_InstanceID, gl_Position, gl_PointSize):

    "Applications may query the location of a built-in attribute by passing its name to getAttribLocation, but this will always return -1."

• MDN
• Chromium Implementation

Root Cause

• JSAR currently returns 0 for built-in attribute names instead of -1, violating spec compliance.
• Correct behavior: gl.getAttribLocation(program, 'gl_VertexID') MUST return -1, never a valid location.

Acceptance Criteria

• getAttribLocation on any built-in attribute ('gl_VertexID', 'gl_InstanceID', 'gl_Position', 'gl_PointSize') MUST always return -1
• The referenced conformance test passes

Implementation Guidance (C++)

• Fix the attribute location query in WebGLRenderingContext/WebGL2RenderingContext
• Check if the attribute name matches any of the built-in names, and always return -1

int WebGLRenderingContext::getAttribLocation(WebGLProgram* program, const std::string& name) {
    static const std::set<std::string> kBuiltInAttribs = {
        "gl_VertexID", "gl_InstanceID", "gl_Position", "gl_PointSize"
    };
    if (kBuiltInAttribs.count(name)) {
        return -1;
    }
    // ... normal lookup logic
}

• Update error handling and unit tests for built-in attribute query

Impact

• Strict compliance with WebGL2 specification
• Enables all Khronos/WebGL2 tests for built-in attrib queries to pass

---

Relevant test HTML:

• https://github.com/EndlessJour9527/WebGL/blob/894d3ba6239ec021a2b65d65d368324c1088aa15/sdk/tests/conformance2/programs/active-built-in-attribs.html
  Spec:
• https://registry.khronos.org/webgl/specs/latest/2.0/#5.18
  Chromium ref:
• https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc;l=3742