feat: GLSLPreviewEngine + GLSL utility functions (#9200)

## Summary

Standalone WebGL2 rendering engine for client-side GLSL shader preview,
with utility functions that mirror the backend `nodes_glsl.py` detection
logic.

## Changes

- **What**: New `GLSLPreviewEngine` class (OffscreenCanvas + WebGL2),
`glslUtils.ts` (detectOutputCount, detectPassCount,
hasVersionDirective), and unit tests
- **GLSLPreviewEngine**: Fullscreen triangle via `gl_VertexID` (no VBO),
ping-pong FBOs for multi-pass rendering, MRT via `gl.drawBuffers()`,
blob output via `canvas.convertToBlob()`
- **glslUtils**: Pure functions ported from backend Python to
TypeScript, regex-based detection matching `_detect_output_count()` and
`_detect_pass_count()`

## Review Focus

- WebGL2 resource lifecycle (context loss, texture cleanup, FBO teardown
in `dispose()`)
- Ping-pong FBO logic for multi-pass shaders
- Engine tests are WebGL2-gated (`describe.skip` in happy-dom) — they
run in real browser environments

## Stacked PR

PR 2 of 3. Stacked on #9198 (fix: GLSLShader preview promotion).
PR 3: `feat/glsl-live-preview` (composable + LGraphNode.vue integration)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9200-feat-GLSLPreviewEngine-GLSL-utility-functions-3126d73d3650812fadc6df4a26387d0e)
by [Unito](https://www.unito.io)

knip.config.ts

@@ -40,6 +40,8 @@ const config: KnipConfig = {
     'packages/registry-types/src/comfyRegistryTypes.ts',
     // Used by a custom node (that should move off of this)
     'src/scripts/ui/components/splitButton.ts',
+    // Used by stacked PR (feat/glsl-live-preview)
+    'src/renderer/glsl/useGLSLRenderer.ts',
     // Workflow files contain license names that knip misinterprets as binaries
     '.github/workflows/ci-oss-assets-validation.yaml',
     // Pending integration in stacked PR