The ornamental design for a seat cushion, as shown and described.
FIG. 1 is a front view of a seat cushion, illustrating my new Design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof; and,
FIG. 4 is a right-side view thereof.
Since the rear view and the left-side view of the new seat cushion design are identical to the front view and the right-side views, respectively these views are omitted.