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

test/api/render_missing.test.cpp

@@ -2,6 +2,7 @@
 #include <mbgl/test/fixture_log_observer.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>
@@ -26,6 +27,7 @@ TEST(API, TEST_REQUIRES_SERVER(RenderMissingTile)) {
     const auto style = util::read_file("test/fixtures/api/water_missing_tiles.json");
 
     HeadlessBackend backend { test::sharedDisplay() };
+    BackendScope scope { backend };
     OffscreenView view { backend.getContext(), { 256, 512 } };
 #ifdef MBGL_ASSET_ZIP
     // Regenerate with `cd test/fixtures/api/ && zip -r assets.zip assets/`