Enable MuJoCo WASM + Filament Rendering Support in Standalone CMake

[MatiasManevi]

Summary (WIP - do not merge)

This PR introduces the character changes required to support Filament-based rendering within the MuJoCo WebAssembly (WASM) bindings using a standalone CMake configuration. This enables a rendering pipeline for the web without a dependency on ThreeJS.

Key changes

• wasm/CMakeLists.txt: Included MuJoCo platform experimental sources (renderer.cc, window.cc, step_control.cc). Configured Emscripten for WebGL2 and ES3 compatibility. Implemented a CMake-native asset compilation and embedding pipeline. Added dependencies for dear_imgui, implot, and libwebp.
• wasm/codegen/embed_assets.py: Developed a custom Python utility to convert binary .filamat and .ktx files into C++ source arrays for direct embedding into the WASM module.

Technical Status & Known Blockers

While the CMake infrastructure is now in place, external/standalone builds currently face a blocker regarding Filament host tools.

1. Filament requires a "Host" build (native) to provide the matc material compiler to the "Target" build (WASM).
2. Users building outside of Google3 must provide -DFILAMENT_HOST_TOOLS_PATH or -DFILAMENT_IMPORT_EXECUTABLES.
3. Initial attempts at a standalone Filament host build identified gaps in the generation of ImportExecutables-Release.cmake in certain upstream versions.

Future work

• Documentation for host-side Filament bootstrap instructions.
• Automated setup for matc and necessary filament tooling.