Reorder waitgroup completion (#1748)

Rather than passing a pointer to a waitgroup into task.Start(..)
functions, handle the waitgroup.Done() at the callsite.

This makes waitgroup handling local to its definition rather than it
being spread out over multiple files. Tasks now simply execute, and
the policy of waiting on them is handled by the caller.

pkg/manager/task_generate_preview.go

@@ -1,8 +1,6 @@
 package manager
 
 import (
-	"github.com/remeh/sizedwaitgroup"
-
 	"github.com/stashapp/stash/pkg/ffmpeg"
 	"github.com/stashapp/stash/pkg/logger"
 	"github.com/stashapp/stash/pkg/manager/config"
@@ -20,9 +18,7 @@ type GeneratePreviewTask struct {
 	fileNamingAlgorithm models.HashAlgorithm
 }
 
-func (t *GeneratePreviewTask) Start(wg *sizedwaitgroup.SizedWaitGroup) {
-	defer wg.Done()
-
+func (t *GeneratePreviewTask) Start() {
 	videoFilename := t.videoFilename()
 	videoChecksum := t.Scene.GetHash(t.fileNamingAlgorithm)
 	imageFilename := t.imageFilename()