Add Transform Feedback manual page

[mvaligursky]

Warning

Do not merge until engine 2.22 is released — the separate-buffer mode documented here ships with that release (playcanvas/engine#9109, playcanvas/engine#9131), as do the diagnostics this page now relies on (playcanvas/engine#9135). The embedded graphics/transform-feedback-separate example does not exist on the live examples site yet, so its thumbnail and iframe currently 404.

Transform feedback was documented nowhere on the site — no mentions in docs/ or i18n/. So this covers both the long-standing interleaved mode and the separate-buffer mode added in 2.22, as a single page.

New page

advanced-rendering/transform-feedback.md, following the structure of the neighbouring Multi-Draw and Hardware Instancing pages:

• Platform Support — WebGL2 only, gated on GraphicsDevice.isWebGL2, with a pointer to compute shaders as the WebGPU equivalent since there is no direct one
• Use Cases — GPU particles, flocking and crowds, cloth, trails
• Live Examples — both engine examples embedded
• A Single Interleaved Buffer — shader, createShader, process(), and why the automatic buffer swap means a mesh rendering from the input buffer needs no updating
• One Buffer Per Varying — TRANSFORM_FEEDBACK_SEPARATE, the three buffer roles, and feeding instanced rendering
• Input Attribute Semantics — declaration-order assignment and the location collision to avoid
• Limitations
• API Reference and Related

Notes worth reviewing

A few points in here are not apparent from the API reference, and were learned the hard way while building the example:

• A buffer cannot be both a per-vertex simulation stream and a per-instance render stream. instancing is a property of VertexFormat, so the divisor applies to every use of the buffer. This is the reason the write-only buffer role exists, so the page explains it rather than just listing the roles.
• Mixing standard and custom attribute names collides. vertex_position maps to location 0, and the first custom name is assigned ATTR0 — also location 0. Called out in a warning admonition.
• Separate mode limits each varying to 4 components on baseline WebGL2, which drives how state gets packed.
  Two further rough edges — a buffer only ever written by the GPU still needing initial data, and BUFFER_GPUDYNAMIC being silently worked around — were filed as TransformFeedback: buffer allocation and usage requirements are implicit engine#9134 and are now reported by the engine itself in Report transform feedback buffer setup problems where they happen engine#9135. Since those asserts name the buffer and say how to fix it, the page does not document the workarounds. The underlying requirements still stand, so if TransformFeedback: buffer allocation and usage requirements are implicit engine#9134 is closed without changing VertexBuffer allocation, it is worth revisiting whether they belong here after all.

Also

• Registered in sidebars.js (the Advanced Rendering list is manual, so the page is invisible without it), placed after Indirect Drawing
• Japanese mirror added with translated frontmatter and English body, matching the convention used by html-in-canvas.md

Testing

• npm run lint — 0 errors across 1306 files
• Rendered locally: breadcrumb resolves as Graphics → Advanced Rendering → Transform Feedback, sidebar entry appears, all 9 sections, the table and the warning admonition render, no console errors
• Both internal links verified against the file tree; compute-shaders lives under shaders/, which caught an initially wrong path
• Both EngineExample placeholders render with the correct titles. The graphics/transform-feedback thumbnail resolves (HTTP 200); graphics/transform-feedback-separate is HTTP 404 until the examples site redeploys — hence the warning above