We claim the ornamental design for a showerbase, as shown and described.
FIG. 1 is a top, front, left side perspective view of a showerbase showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
Broken lines of the disclosure represent portions of the showerbase that form no part of the claimed design. The appearance of any portion of the article between the broken away symbols forms no part of the claimed design.