Support for layer stacks

[dnicolson]

It appears that on tvOS the primary icon is a layer stack that is associated with the ZZZZFlattenedImage-1.1.0-gamut0
ZZZZRadiosityImage-1.0.0 images, the "App Icon" images are used as a fallback.

Here is an example:

This is the log output when using AssetCatalogWrapper directly:

CoreUI: Unable to resolve layer reference for '<CUIRenditionLayerReference: 0x600002287890>' name 'App Icon' layerRef referenceKey '<CUIRenditionKey: 0x600002ab2d80>
element: 85,
part: 181,
size: 0,
direction: 0,
value: 0,
dimension1: 0,
dimension2: 0,
state: 0,
presentation: 0,
previousState: 0,
previousValue: 0,
layer: 0,
scale: 1,
gamut: 0
identifier: 59864
memoryClass 0
graphicsClass: 0
deployment: 0
appearance identifier: 0 
localization identifier: 0 
glyph size: 0 
glyph weight: 0'
CoreUI: Unable to resolve flattened image for layer stack <CUINamedLayerStack: 0x600002287840>
CoreUI: Unable to resolve radiosity image for layer stack App Icon