The ornamental design for a mat, as shown and described.
FIG. 1 is a perspective view of the mat showing my new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a front side view thereof; and,
FIG. 4 is a bottom plan view thereof, oriented such that the "lower
right-hand corner " of the mat shown in FIG. 1 corresponds to the "upper
right-hand corner" of the mat shown in FIG. 4.
The left side view of the mat is identical to the right side view of the
mat, the back view is identical to the front view, and the bottom plan
view is identical to the top plan view.