Don't apply modifications from multiple items with vision magnification

Currently characters with 2 instances of vision_magnification (e.g. one in a helmet and one in glasses) get twice the attack rating benefit (+4 instead of +2). This needs to be corrected to only +2, no matter how ofter vision_magnification is available on the character.

Update: It seems that vision magnification is applied to “installed weapons” (e.g. underbarrel grenade launcher of a Raiden) every time the character is loaded. Thus the attack rating of those weapons continously increases.