Conditionally utilize VK_EXT_external_memory_host to optimize bandwidth

[Wunkolo]

This is something I've already done before in my other personal plugins, where rather than copying data into a staging ring-buffer to upload/download onto the GPU, VK_EXT_external_memory_host allows arbitrary pointers to be imported directly. This would remove an entire copy from the upload/download path by allowing vulkan to directly write into host-memory.

This would also have to introduce the concept of "trivially importable memory", where minImportedHostPointerAlignment is the same as the operating system's page-size (4096 bytes) allowing practically any pointer to be imported.

MoltenVK support is tracked over at KhronosGroup/MoltenVK#794.

[Wunkolo]

MacOS dependency on KhronosGroup/MoltenVK#794 and KhronosGroup/MoltenVK#1854