The ornamental design for a cushion, substantially as shown and described.
FIG. 1 is a perspective view of the front side and a left hand side of a cushion showing my new design;
FIG. 2 is top view of the cushion shown in FIG. 1, with the bottom view being identical to the top view;
FIG. 3 is front elevation of the cushion shown in FIG. 1, with the rear elevation being identical to the front elevation; and,
FIG. 4 is right hand side elevation thereof of the cushion shown in FIG. 1, with the left hand side elevation being identical to the right hand side elevation.