Manages the lifetime of the on-screen and off-screen rendering contexts on iOS. On-screen contexts are used by Flutter for rendering into the surface. The lifecycle of this context may be tied to the lifecycle of the surface. On the other hand, the lifecycle of the off-screen context it tied to that of the platform view. This one object used to manage both context because GPU handles may need to be shared between the two context. To achieve this, context may need references to one another at creation time. This one object manages the creation, use and collection of both contexts in a client rendering API agnostic manner