Fix sprite bound issues for vehicles which vary bounds with curvature

If a vehicle uses curvature vars 0x45 or 0x62, cache the directions
of its 4 nearest neighbours, and check them in UpdateViewport along
with the vehicle's own direction.

src/newgrf_cache_check.h

@@ -11,5 +11,6 @@
 #define NEWGRF_CACHE_CHECK_H
 
 extern bool _sprite_group_resolve_check_veh_check;
+extern bool _sprite_group_resolve_check_veh_curvature_check;
 
 #endif /* NEWGRF_CACHE_CHECK_H */