feat(live2d): implement OpenGL ES 2.0 rendering backend with EGL support

- Add `gl.rs` for OpenGL ES 2.0 and EGL context management.
- Introduce `mod.rs` to organize Live2D plugin modules.
- Create `model.rs` for loading and managing Cubism models, including parameter handling and drawable data extraction.
- Implement shader compilation and rendering context setup for Live2D models.

.gitignore

@@ -3,6 +3,7 @@ target/
 *.swp
 *.swo
 *~
+extern/
 # 模型文件不进 git，只保留下载链接在 model_manager.rs 清单里
 model_download/
 model_store/