Refactored multiple cameras for render targets. Adding setting of camera size

Camera render targets now work when using multiple cameras. Setting the
size of a camera is now possible anytime after the camera has been
created, so this allows the initial game camera to be resized.
Individual camera viewports are no longer needed and have been removed.
Cameras now follow the center point of a follow target, whereas they
would previously follow the top-left corner of the target object.