Multi-threaded resource loading

[Hasan-Jawaheri]

Need support for loading assets from worker thread(s).

Functional requirements:

• Data should be loadable separately from Vulkan-specific resources.
• An estimate on the loading time should be made based on the size/complexity of the data.
• Loaded data should be made available in Vulkan resources during the game loop slowly and per frame. The number of resources that can be processed by Vulkan per loop iteration should depend on the estimated loading times.
• There should be a user-specified priority to loading such that high-priority resources are loaded ASAP.
• Dependencies should be properly handled.
• Rules for custom (user-defined) dependencies may be required, in the form of callbacks on-load.
• All loading must be done this way. Application should be able to wait on loading to happen while running a splash screen or playing a mini-game.