title: Vulkan: Difference between instance and device extensions?
body: <p>Vulkan has <em>instance</em> and <em>device</em> extensions, but I cannot find any information anywhere about what the difference between them is. What does it mean exactly if something is a device extension or an instance extension? Why is <a href="https://www.khronos.org/registry/vulkan/specs/1.0-extensions/html/vkspec.html#VK_KHR_external_memory" rel="noreferrer">VK_KHR_external_memory</a> a device extension and <a href="https://www.khronos.org/registry/vulkan/specs/1.0-extensions/html/vkspec.html#VK_KHR_external_memory_capabilities" rel="noreferrer">VK_KHR_external_memory_capabilities</a> an instance extension? Why is it not just a single, unified extension system?</p>