[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.

test/map/map.test.cpp

@@ -316,7 +316,7 @@ TEST(Map, DefaultBoundOptions) {
 
     EXPECT_EQ(*bounds.minZoom, util::MIN_ZOOM);
     EXPECT_EQ(*bounds.maxZoom, util::DEFAULT_MAX_ZOOM);
-    EXPECT_EQ(*bounds.bounds, LatLngBounds::unbounded());
+    EXPECT_EQ(*bounds.bounds, LatLngBounds());
 }
 
 TEST(Map, MapOptions) {
@@ -1053,4 +1053,4 @@ TEST(Map, NoHangOnMissingImage) {
     test.map.jumpTo(test.map.getStyle().getDefaultCamera());
     // The test passes if the following call does not hang.
     test.frontend.render(test.map);
-}
+}