[Refactor] (CMakeLists.txt, MG_Backend, MG_Backend/DirectGLES, MG_Backend/DirectVulkan, MG_Remote/Client, MG_Remote/Protocol, MG_Remote/Server, MG_Test/Backend, scripts/data): move BackendObject's EGL bookkeeping into EGLSurfaceRegistry

- EGLSurfaceRegistry owns the display, the surface records, the active surface, each thread's binding and the one recursive lock; BackendObject holds one and is its hooks, so the EGL virtuals keep their signatures and their defaults
- the backends reach it through EGLRegistry(): lookups return copies, and the steps they compose (adopting an already-bound surface, a window's surfaces, a window's new extent) are named operations instead of writes to protected fields
- WindowBackend and WindowHandle move with it
- BackendObjectEglStateTest only re-points its accessors; the same 235 mutants, carried over, get the same verdicts except three that need a kind-None record, which no call can make, and one that cannot be written against the registry
- comments that cited BackendObject.cpp line numbers name the function

MobileGL/MG_Remote/Protocol/SurfaceOpCodec.cpp

@@ -82,7 +82,7 @@ namespace MobileGL::MG_Remote {
         static_assert(static_cast<Uint8>(SurfaceRefusalCode::ServerOwnedOnSetWindowHandle) ==
                       static_cast<Uint8>(::MobileGL::Wire::SurfaceRefusal::ServerOwnedOnSetWindowHandle));
         // The frame-local ServerOwned tag names no WindowBackend, by construction (D2: no
-        // BackendObject.h enumerator for it - that header is in the pull build, gate G1).
+        // EGLSurfaceRegistry.h enumerator for it - that header is in the pull build, gate G1).
         static_assert(Server::kServerOwnedWindowBackend >=
                           static_cast<Int>(MG_Backend::WindowBackend::WindowBackendCount),
                       "kServerOwnedWindowBackend collides with a real WindowBackend");