The ornamental design for a shower mixer, as shown and described.
FIG. 1 is a front perspective view of a shower mixer according to the present invention.
FIG. 2 is a left side view of the shower mixer of FIG. 1.
FIG. 3 is a right side view of the shower mixer of FIG. 1.
FIG. 4 is a top view of the shower mixer of FIG. 1.
FIG. 5 is a bottom view of the shower mixer of FIG. 1.
FIG. 6 is a front view of the shower mixer of FIG. 1; and,
FIG. 7 is a rear view of the shower mixer of FIG. 1.
The dashed lines illustrate environment that does not form a part of the present invention, and no claim is made to the material illustrated with dashed lines.