fix: Avoid slices.Contains within tight loops

Having a `slices.Contains` check within a `for` loop has a complexity of
`O(n*m)`, with a worst case scenario of `O(n^2)` for when both
collections have the same size.

This is because `slices.Contains` internally runs just a regular loop,
checking for equality element by element.

Instead, this diff changes every occurence of this pattern to converting
the collection we compare against to a set, and then running a faster
lookup against it.

sharedutil/sharedutil.go

@@ -98,8 +98,9 @@ func ReorderTracks(tracks []*mediaprovider.Track, idxToMove []int, op TrackReord
 	case MoveToTop:
 		topIdx := 0
 		botIdx := len(idxToMove)
+		idxToMoveSet := ToSet(idxToMove)
 		for i, t := range tracks {
-			if slices.Contains(idxToMove, i) {
+			if _, ok := idxToMoveSet[i]; ok {
 				newTracks[topIdx] = t
 				topIdx++
 			} else {
@@ -110,8 +111,9 @@ func ReorderTracks(tracks []*mediaprovider.Track, idxToMove []int, op TrackReord
 	case MoveToBottom:
 		topIdx := 0
 		botIdx := len(tracks) - len(idxToMove)
+		idxToMoveSet := ToSet(idxToMove)
 		for i, t := range tracks {
-			if slices.Contains(idxToMove, i) {
+			if _, ok := idxToMoveSet[i]; ok {
 				newTracks[botIdx] = t
 				botIdx++
 			} else {