Camera Groups or: Let cameras share vibility info

[klepto2]

This is a feature only a minority will ever need, but can increase the performance a lot in some cases:

• custom refraction / reflection
• posteffects which are meant as an overlay or backdrop

What i mean exactly:

Currenty each camera performs its own culling, even when the camera shares the same attributes as postion, rotation and range. My idea is to add a property to a camera where you can assign a main or parent camera. Now the main or parent camera performs the culling and the "child" cameras just consume the same visibility lists or data as the main camera. Even when pure gpu culling will be introduced this will increase performance as it can save some gpu processing which can be used for other things.