Physical device (Vulkan terminology) or adapter (D3D/DXGI terminology) selection where applicable: what do we want to support?

• Currently we do have env.vars. like QT_D3D_ADAPTER_INDEX and QT_VK_PHYSICAL_DEVICE_INDEX.

• Should there be something less backend specific and more general? We have to be careful with introducing full-blown adapter enumeration and management APIs, however. Think about what a simple, compact API would look like.

• Metal: do we care about choosing between MTLDevices, or just using the "system default" is enough? (do not have any hardware with multiple GPUs, cannot investigate)

• Do we need to monitor the device list on Metal? Only relevant on macOS with external GPUs (disconnect at runtime - apps relying on that GPU are quit/restarted by default by the OS, I think, perhaps that good enough?) Cannot test, no hardware available.