Handle initial actor's transparency

[akortunov]

Fixes bug #4860.

The main idea:

1. Move visibility update to the separate function
2. Call it after adding actor to scene as well during actor's update.

Note: when we teleport player, we can not check distance to actor since player still has old coordinates.
I do not know how to handle this case - for now I just ignore the visibility check.

[psi29a]

I'll take it, thank you.