Implement WebGL2 transformFeedbackVaryings API

[Copilot]

WebGL2 applications calling gl.transformFeedbackVaryings() fail with "Not implemented" error, breaking conformance tests and libraries (Three.js, Babylon.js) that use transform feedback for GPU computation and particle systems.

Changes

Command buffer infrastructure (src/common/command_buffers/)

• Added TRANSFORM_FEEDBACK_VARYINGS command type
• Created TransformFeedbackVaryingsCommandBufferRequest with string array serialization

Client implementation (src/client/graphics/)

• Added WebGL2Context::transformFeedbackVaryings() dispatcher
• Validates program and sends command buffer request

JavaScript binding (src/client/script_bindings/webgl/)

• Implemented WebGL2RenderingContext::TransformFeedbackVaryings()
• Validates 3 required arguments (WebGLProgram, string[], GLenum)
• Extracts varying names from JS array

Renderer handler (src/renderer/)

• Implemented OnTransformFeedbackVaryings()
• Converts C++ string vector to C-style char** array
• Calls glTransformFeedbackVaryings() with proper error checking

Usage

const program = gl.createProgram();
// ... attach and link shaders ...

gl.transformFeedbackVaryings(
  program, 
  ['gl_Position', 'vColor'], 
  gl.INTERLEAVED_ATTRIBS
);

Conforms to WebGL 2.0 §5.19.3.

Original prompt

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This section details on the original issue you should resolve

<issue_title>WebGL2: transformFeedbackVaryings API not implemented, breaks conformance and library support</issue_title>
<issue_description>## Background
When running WebGL2 code or Khronos conformance tests in JSAR Runtime, the following error is encountered:

Uncaught Error: Failed to execute 'transformFeedbackVaryings' on 'WebGL2RenderingContext': Not implemented

This occurs when JS/WebGL2 code calls the API:

gl.transformFeedbackVaryings(program, ['gl_Position'], gl.INTERLEAVED_ATTRIBS);

Impact

• Standard conformance tests requiring transform feedback fail to run in JSAR
• Popular libraries that use transform feedback (e.g., Three.js advanced rendering patterns, Babylon.js particle systems, GPU-based computation, etc.) fail or degrade
• Both classic and XR spatial apps depending on this API are blocked

Web Standard Reference

• WebGL 2.0 Specification §5.19.3 transformFeedbackVaryings
• Chromium implementation (WebGL2RenderingContextBase::TransformFeedbackVaryings)
• MDN docs - transformFeedbackVaryings

Root Cause

• The native C++ binding for WebGL2RenderingContext.transformFeedbackVaryings is missing or stubbed as "not implemented" in JSAR
• No logic dispatching from JS layer to the actual OpenGL call (glTransformFeedbackVaryings)

Acceptance Criteria

• Calling gl.transformFeedbackVaryings in JSAR WebGL2 context correctly dispatches to native and configures transform feedback as per spec
• Conformance tests referencing this API pass
• Popular libraries relying on transform feedback work (Three.js, Babylon.js examples)

Implementation Guidance (C++)

• Add JS binding for transformFeedbackVaryings in WebGL2RenderingContext
• Dispatch to corresponding OpenGL API:

void WebGL2RenderingContext::transformFeedbackVaryings(WebGLProgram* program, const std::vector<std::string>& varyings, GLenum bufferMode) {
    // Convert string array to C array
    std::vector<const char*> c_varyings;
    for (const auto& v : varyings) {
        c_varyings.push_back(v.c_str());
    }
    glTransformFeedbackVaryings(program->glObject(), c_varyings.size(), c_varyings.data(), bufferMode);
}

• Update JS-to-native binding so the method is available on WebGL2RenderingContext
• Fails gracefully (throws per spec) if invoked on invalid program object

Relevant code location to update:

• src/client/builtin_scene/webgl_rendering_context.cpp

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Supporting this API is required for WebGL2 spec conformance and ecosystem compatibility.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes WebGL2: transformFeedbackVaryings API not implemented, breaks conformance and library support #410

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