Move OpenGL extension query into renderer backend

docs/modernization/build-inventory.md

@@ -791,7 +791,9 @@ Known warnings after the current CMake app build:
   `pp_renderer_gl` now owns OpenGL runtime build-target classification through
   CMake target compile definitions and `opengl_runtime_for_current_build()`,
   so app shader startup asks the backend for desktop GL/GLES/WebGL policy
-  instead of carrying local platform branches.
+  instead of carrying local platform branches. It also owns headless-tested
+  OpenGL extension enumeration through `query_opengl_extensions`, moving the
+  extension count/string query loop out of `app_shaders.cpp`.
 - `pp_legacy_ui_core` is an object-library containment boundary because the
   retained base `Node` controls still depend on legacy renderer and app
   headers. It should shrink as layout parsing, colors, generic controls, and