An ornamentaldesign for a support cushion, as shown and described.
FIG. 1 is a perspective view showing the top, front and sides of the support cushion of my new design;
FIG. 2 is a front view of my new design for the support cushion;
FIG. 3 is a side view of my new design for the support cushion;
FIG. 4 is a top view of my new design for the support cushion; and,
FIG. 5 is bottom view of my new design for the support cushion.
The outer perimeter of broken line in FIG. 5 represents the boundary of the claimed design. The broken line features within the broken line boundary form no part of the claim.