The ornamental design for packaging casing, as shown and described.
FIG. 1 is a front view of a packaging casing of the present invention;
FIG. 2 is a top perspective view thereof, shown at an enlarged scale;
FIG. 3 is a bottom perspective view thereof;
FIG. 4 is a top plan view thereof, shown at a further enlarged scale; and,
FIG. 5 is a bottom plan view thereof.
The portions of the design shown in broken lines form no part of the claimed design. The horizontal lines on the surfaces of the packaging casing designate the color blue.