fix crash when context is cancelled when creating gif

Semaphore acquisition will still succeed without modifying the semaphore
itself when the context is cancelled. This creates a situation where it
is possible to release more semaphores than held.

ffmpeg/ffmpeg.go

@@ -143,6 +143,10 @@ func (f *Ffmpeg) MakeGifThumbnail(ctx context.Context, videoUrl string, outputFi
 	}
 	defer f.doneProcessing(videoUrl)
 
+	// Disconnect from the context
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
 	// Create temporary directory
 	workdir, err := os.MkdirTemp("", "thumb-")
 	if err != nil {