common: SlotVector without generations

src/graphics/host_gpu/renderer/image/image.h

@@ -139,7 +139,10 @@ public:
 	}
 
 	static inline std::atomic<uint64_t> heap_epoch {0};
+	static inline std::atomic<uint64_t> serials {0};
 
+	// Unique per image, unlike ImageId: freed slots are reused.
+	const uint64_t   serial = serials.fetch_add(1u, std::memory_order_relaxed) + 1u;
 	ImageInfo        info;
 	VulkanImage      backing;
 	std::vector<CachedImageView> views;