Add support for VK_KHR_external_memory for inter-device memory access (depends on VK_KHR_external_memory_capabilities)

[nanokatze]

https://www.khronos.org/registry/vulkan/specs/1.0-extensions/html/vkspec.html#VK_KHR_external_memory

An application may wish to reference device memory in multiple Vulkan logical devices or instances, in multiple processes, and/or in multiple APIs. This extension enables an application to export non-Vulkan handles from Vulkan memory objects such that the underlying resources can be referenced outside the scope of the Vulkan logical device that created them.

The different logical devices part makes it possible for Vulkano users to implement multi-device renders etc without performing on-CPU inter-device buffer copies which are only suitable in very few cases and impose high load on CPU.

[nanokatze]

I think this should be low priority

[gurchetansingh]

For those still interested, here's the first stab at this:

https://github.com/vulkano-rs/vulkano/pull/1467

[Rua]

This is now more thoroughly implemented, although there's still room for adding more platforms and memory types.