fix: Camera USB stack crash after second capture

[fzorbach]

I am using the Sony Alpha 7s.
I experienced an issue when taking a picture. gp_camera_capture returned GP_ERROR_IO when taking the second picture (after start of the app). Retrying will then return GP_ERROR_IO_USB_FIND. The photobox app now hangs in a loop right here.

                       | exec::repeat_effect_until(); // todo: this now retries forever to capture a image.
                                                      // maybe retry_n times and emit an error?

In the mean time, the camera will disappear from the USB devices. I guess the USB stack in the camera errored out.

The camera does not recover unless restarted.

After fiddling around, i noticed that nothing calls gp_camera_wait_for_event. So I had a feeling that the camera had some pending events it wanted to process before taking another picture. Calling gp_camera_wait_for_event until GP_EVENT_TIMEOUT occurs does fix it.

The way I implemented it should wait for the event while the captured image is shown. So no preview frames are lost and no further delay should occur. Right now, 6 events occur before timeout (1x file added, 2x capture complete, 3x unknown).

Due to not having more cameras, I can only test it with the Alpha 7s, which works fine.

Open for discussion on how to integrate that properly. I've never worked with stdexec before.