Support for multiple composition layers, additional composition layer types

[geefr]

Needed for nice display of UI panels/quads, skyboxes, etc.

Prototype in composition-layers branch, this will need a rework to the viewer setup, swapchain creation, etc,

[geefr]

Quads implemented - Skybox and similar need additional composition layers, as optional extensions

[geefr]

KHREquirect and KHREquirect2 (extension) layers now present.

Displaying any RenderGraph on a composition layer also now possible - Need to demo a RenderGraph for a simple image copy or similar.

Imgui worked for proof of concept, in vsgimgui-prototype branch -> Will need to make vsgimgui a bit more flexible for the display side there, and add some helpers for mapping intersections to mouse input etc.