Additional frustum culling events in order to better support indirect drawing

[mvaligursky]

This PR:

• fires existing EVENT_PRECULL and EVENT_POSTCULL events even for shadow cameras
• fires new EVENT_CULL_END event when all culling is completed (scene + shadow cameras)

This along with Indirect Drawing can be used to implement instanced culling on GPU:

1. Collect all cameras the scene uses for rendering of the frame (scene + shadow cameras)
2. dispatch a Compute shader which culls instanced mesh instances, and uses indirect drawing to render visible instances per camera