[core] Ensure that a BackendScope exists when doing GL calls

test/util/memory.test.cpp

@@ -3,6 +3,7 @@
 #include <mbgl/test/util.hpp>
 
 #include <mbgl/map/map.hpp>
+#include <mbgl/map/backend_scope.hpp>
 #include <mbgl/gl/headless_backend.hpp>
 #include <mbgl/gl/offscreen_view.hpp>
 #include <mbgl/util/default_thread_pool.hpp>
@@ -35,6 +36,7 @@ public:
 
     util::RunLoop runLoop;
     HeadlessBackend backend { test::sharedDisplay() };
+    BackendScope scope { backend };
     OffscreenView view{ backend.getContext(), { 512, 512 } };
     StubFileSource fileSource;
     ThreadPool threadPool { 4 };