avoid hang on closing webcam due to rescans for new camera devices

src/main/java/com/sparrowwallet/sparrow/control/WebcamScanDriver.java

@@ -8,8 +8,11 @@ import java.util.ArrayList;
 import java.util.List;
 
 public class WebcamScanDriver extends WebcamDefaultDriver {
+    private List<WebcamDevice> foundScanDevices;
+
     @Override
     public List<WebcamDevice> getDevices() {
+        if(foundScanDevices == null || foundScanDevices.isEmpty()) {
             List<WebcamDevice> devices = super.getDevices();
             List<WebcamDevice> scanDevices = new ArrayList<>();
             for(WebcamDevice device : devices) {
@@ -17,6 +20,9 @@ public class WebcamScanDriver extends WebcamDefaultDriver {
                 scanDevices.add(new WebcamScanDevice(defaultDevice.getDeviceRef()));
             }
 
-        return scanDevices;
+            foundScanDevices = scanDevices;
+        }
+
+        return foundScanDevices;
     }
 }