vkd3d: Store a struct d3d12_descriptor_heap pointer in each CBV/SRV/UAV descriptor.
The PE build of vkd3d uses Windows critical sections for synchronisation, and these slow down on the very high lock/unlock rate during multithreaded descriptor copying in Shadow of the Tomb Raider. This patch speeds up the demo by about 8%. By comparison, using SRW locks in the allocators and locking them for read only where applicable is about 4% faster.