An optional Rectangle object that is used during rendering to cull Particles from display. For example, if your particles are limited to only move within a 300x300 sized area from their origin, then you can set this Rectangle to those dimensions.
The renderer will check to see if the viewBounds Rectangle intersects with the
Camera bounds during the render step and if not it will skip rendering the Emitter
entirely.
This allows you to create many emitters in a Scene without the cost of rendering if the contents aren't visible.
Note that the Emitter will not perform any checks to see if the Particles themselves
are outside of these bounds, or not. It will simply check the bounds against the
camera. Use the getBounds method with the advance parameter to help define
the location and placement of the view bounds.