Do not process vision for remote players when they are between dungeon levels

[StephenCWills]

When the remote player transitions from their current level to the loading screen, the local client...

• Receives CMD_NEWLVL
• Calls player.setLevel() to update player.plrlevel
• Sets player._pLvlChanging to true

When the remote player transitions from the loading screen to their new level, the local client...

• Receives CMD_PLAYER_JOINLEVEL
• Sets player._pLvlChanging to false
• Calls StartStand() which calls FixPlayerLocation() to update the player's position
• Calls ActivateVision() on the remote player instance

When the local player transitions from their current level to the loading screen after processing CMD_NEWLVL but before processing CMD_PLAYER_JOINLEVEL, the player.isOnActiveLevel() function will return true for the remote player because player.setLevel() was already called, but the remote player's location won't have been updated because FixPlayerLocation() hasn't been called.

This means that when LoadGameLevelLightVision() calls ProcessVisionList(), it will call DoVision() on the remote player in the wrong location. Since we know the local client will eventually call ActivateVision() on the remote player instance when CMD_PLAYER_JOINLEVEL is processed, it's okay to keep the remote player's vision deactivated while player._pLvlChanging is true.

This resolves #7745