Add option to generate image thumbnails during generate (#4602)

* Add option to generate image thumbnails
* Limit number of concurrent image thumbnail generation ops

internal/manager/manager.go

@@ -10,6 +10,7 @@ import (
 	"runtime"
 	"time"
 
+	"github.com/remeh/sizedwaitgroup"
 	"github.com/stashapp/stash/internal/dlna"
 	"github.com/stashapp/stash/internal/log"
 	"github.com/stashapp/stash/internal/manager/config"
@@ -33,6 +34,10 @@ type Manager struct {
 	Config *config.Config
 	Logger *log.Logger
 
+	// ImageThumbnailGenerateWaitGroup is the global wait group image thumbnail generation
+	// It uses the parallel tasks setting from the configuration.
+	ImageThumbnailGenerateWaitGroup sizedwaitgroup.SizedWaitGroup
+
 	Paths *paths.Paths
 
 	FFMpeg        *ffmpeg.FFMpeg
@@ -107,6 +112,8 @@ func (s *Manager) RefreshConfig() {
 		if err := fsutil.EnsureDir(s.Paths.Generated.InteractiveHeatmap); err != nil {
 			logger.Warnf("could not create interactive heatmaps directory: %v", err)
 		}
+
+		s.ImageThumbnailGenerateWaitGroup.Size = cfg.GetParallelTasksWithAutoDetection()
 	}
 }