Use Vulkan Memory Allocator (VMA) from the Vulkan SDK

VMA is included in the most recent Vulkan SDKs! We should (probably?!) use that version instead of the one which comes bundled with this repository: see file [vk_mem_alloc.h](https://github.com/cg-tuwien/Auto-Vk/blob/master/include/vk_mem_alloc.h)

Make the necessary configuration changes!