This MR implements memory mapping for wow64 using VK_EXT_external_memory_host. This has some problems (more below), but overall, it looks promising as a stopgap. It passes D3D tests and works fine in general.
For proper solution, we'd need a new Vulkan extension. I experimented with a prototype extension https://github.com/KhronosGroup/Vulkan-Docs/pull/1906 and it looks like we can make it work, my WIP implementation is here https://gitlab.winehq.org/jacek/wine/-/tree/vulkan-map-placed. However, it would be nice to have a fallback working on existing drivers as well.
The main problem of this MR is that it breaks Vulkan spec. The main rule it breaks is that according to spec, we should explicitly specify that we plan to use external memory when creating VkBuffer or VkImage. The problem is that we don't know if we will until we bind memory to the buffer. I tried some workarounds for that (delaying actual buffer creation, recreating buffer when needed). I got it mostly working and passing tests without validation errors, but it was more invasive than I'd like and still had other problems. I'd rather avoid that if possible.
After looking at Mesa sources, I think that the way this MR breaks the spec should be harmless for its drivers. I don't have the hardware to test with Nvidia. If it works there, I'm tempted to ignore spec violation for now, given that the right path forward is VK_EXT_map_memory_placed anyway.
Performance-wise, this MR is sub-optimal because we're limited in what memory types we can use. However, I ran Unigine benchmarks on radv and the difference was marginal. In case of Heaven, it was even slightly faster on wow64, which could be explained by 64-bit Vulkan driver being generally slightly faster, but the difference is too small to draw any conclusion.
The last patch is not meant for (eventual) merge, but it's nice for testing the concept. It makes it possible to force using external memory in any kind of Wine build to experiment with its effects without need for wow64. To test that, just set `WINEVULKAN_EXTERNAL_MEMORY` environment variable.