Move OpenGL runtime classification into renderer backend

docs/modernization/build-inventory.md

@@ -788,6 +788,10 @@ Known warnings after the current CMake app build:
   the retained OpenGL runtime classes still include legacy app/engine headers
   and are still consumed directly by canvas and UI classes. It should become a
   normal backend library once those call sites depend on `pp_renderer_api`.
+  `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.
 - `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