ImageCopyCaptureSession broken on labwc

Describe the bug When using w0vncserver on Labwc, you just get a No Wayland pixel formats provided error when the client connects.

To Reproduce Steps to reproduce the behavior:

1. Start w0vncserver
2. Connect with client
3. Connection fails, server terminates connection.

Server (please complete the following information):

• OS: Fedora 44
• VNC server: TigerVNC
• VNC server version: master
• Server was started using: w0vncserver -log *:stderr:100

Additional context The error is caused by receiving two handleDone() calls in ImageCopyCaptureSession, which resets our formatsPending. Between the first and the second handleDone(), there are no new formats advertised.

We probably shouldn't reset the formats when we run handleDone().

Weirdly enough, we did not encounter this on Labwc when this was developed. I have not tested any other compositors.