Add MVKPresentableSwapchainImage and MVKPeerSwapchainImage subclasses to MVKSwapchainImage, with MVKPresentableSwapchainImage instances created inside swapchain, and MVKPeerSwapchainImage instances created using vkCreateImage(). MVKPeerSwapchainImage retrieve and share CAMetalDrawable from corresponding MVKPresentableSwapchainImage.