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

test/api/annotations.test.cpp

@@ -5,6 +5,7 @@
 #include <mbgl/annotation/annotation.hpp>
 #include <mbgl/sprite/sprite_image.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/io.hpp>
@@ -24,6 +25,7 @@ class AnnotationTest {
 public:
     util::RunLoop loop;
     HeadlessBackend backend { test::sharedDisplay() };
+    BackendScope scope { backend };
     OffscreenView view { backend.getContext() };
     StubFileSource fileSource;
     ThreadPool threadPool { 4 };