When multiple mixed directional lights are used in the scene, shadowmask fails to fade between realtime and baked shadows correctly in forward rendering path.

Steps for reproduction:
1) Download the project `Mixed Directional` and open the scene `Scene.unity`
2) Ensure that rendering path of camera is set to Forward
3) Either zoom in and out in the scene view or adjust the shadow distance
4) Observe that realtime and baked shadows fail to blend properly

Notes:
- This is only a problem in forward rendering path, works correctly in deferred
- A gif that demonstrates the problem is attached below