Allow P_CheckSight to see through FF_FOG FOFs

What it says on the tin. More specifically though, it's the walls of these FOFs that block enemies or bosses from seeing you; when everyone is in the same sector the walls don't block their view of you at all.

Probably needs testing to make sure I didn't break anything in a vanilla SRB2 level with fog somehow by doing this. Otherwise, this is a pretty simple fix.