VK_EXT_descriptor_buffer support

[ishitatsuyuki]

VK_EXT_descriptor_buffer is expected to be used by and provide some performance boost for vkd3d-proton.

This extension has some tricky aspects, mainly the use of direct memory writes which requires similar handling as HOST_VISIBLE buffer writes.

[bradgrantham-lunarg]

Does vkd3d-proton work without this extension?

[ishitatsuyuki]

It can fallback to the traditional binding model, however it’s a drastically different code path, with things such as buffer view offset buffer (16B per SRD, required because views in D3D12 are just the descriptors themselves and creating individual VkBufferViews would insanely complicate lifetime tracking). These buffers also lead to very different performance characteristics, since they consume a considerable amount of non-resizable BAR budget (64M out of 256M) and pushes out other upload heaps to system memory.