Rendering a Scene multiple times for split screen with PostProcessing enabled is broken

[fallahn]

By default a Scene can be rendered multiple times with different Camera components to create effects such as split screen or picture in picture. This breaks when PostProcess is enabled as the post process buffer gets cleared between draw calls. The fix for this is to submit a list of Camera component carrying entities to the Scene when drawing - see the crogine repository for potential implementation.

[fallahn]

This also affects the culling of UIHitboxes in UISystem