Vulkan has different descriptor types for buffers and textures.
Therefore, we do not know the exact Vulkan descriptor type for D3D12 SRV
and UAV descriptors up front. This information can be extracted from
shaders when creating PSOs but creating incompatible Vulkan descriptor
set layouts for PSOs with the same root signature would introduce a lot
of complexity.
In order to preserve compatibility between resource bindings for PSOs
with the same root signature we duplicate Vulkan descriptors for SRVs
and UAVs (a buffer view and an image view descriptor). This strategy may
be reasonable for small root signatures.
Direct3D 12 resources, buffers in particular, are bound to the pipeline
by their GPU virtual address. In Vulkan, these addresses are not visible
to the application. We previously handled this by returning the VkBuffer
handle as virtual address, but this can't work when the application
binds anything other than the resource's base GPU VA. Instead, we
allocate ranges of GPU address space and associate resources with it.
This uses the (naive) approach of simply allocating subsequent ranges,
and never reclaiming them. Eventually we'll have to revisit that.
