[core] Fix MapSnapshotter build failure on Windows

MSVC implementation of std::promise is buggy and only works with types
that can be default-constructed. To avoid a compilation failure in the
instantiation of ask() inside MapSnapshotter::getRegion(), which creates
a std::promise<LanLngBounds>, make LatLngBounds' default constructor
public.

platform/android/src/native_map_view.cpp

@@ -314,7 +314,7 @@ void NativeMapView::setLatLngBounds(jni::JNIEnv& env, const jni::Object<mbgl::an
     if (jBounds) {
         bounds.withLatLngBounds(mbgl::android::LatLngBounds::getLatLngBounds(env, jBounds));
     } else {
-        bounds.withLatLngBounds(mbgl::LatLngBounds::unbounded());
+        bounds.withLatLngBounds(mbgl::LatLngBounds());
     }
     map->setBounds(bounds);
 }