Report USB device disconnection when detected

USB device disconnection is detected via a hotplug callback when it is
supported.

In addition, report disconnection on libusb calls returning
LIBUSB_ERROR_NO_DEVICE or LIBUSB_ERROR_NOT_FOUND. This allows to detect
disconnection after a libusb call when hotplug is not available.

PR #3011 <https://github.com/Genymobile/scrcpy/pull/3011>

app/src/usb/usb.h

@@ -22,6 +22,7 @@ struct sc_usb {
     sc_thread libusb_event_thread;
 
     atomic_bool stopped; // only used if cbs != NULL
+    atomic_flag disconnection_notified;
 };
 
 struct sc_usb_callbacks {
@@ -73,6 +74,11 @@ sc_usb_connect(struct sc_usb *usb, libusb_device *device,
 void
 sc_usb_disconnect(struct sc_usb *usb);
 
+// A client should call this function with the return value of a libusb call
+// to detect disconnection immediately
+bool
+sc_usb_check_disconnected(struct sc_usb *usb, int result);
+
 void
 sc_usb_stop(struct sc_usb *usb);