The ornamental design for a cushion, as shown and described.
FIG. 1 is an isometric view of a cushion;
FIG. 2. is a top view (plan) of the cushion;
FIG. 3. is a front view of the cushion; and,
FIG. 4. is a rear isometric view of the cushion.
The design is to be applied to a cushion and the novelty resides in the features of shape and/or configuration of the cushion as shown in the accompanying representations.