theme: Make sure to clear the entirety of buttons properly

In commit cc5def10, buttons were changed from GdkRectangles to
MetaButtonSpace units, but the corresponding memset hack was not.

This means that the clickable portion of the unshade rectangle
was always set to uninitalized memory. The effects of this were
random, but in cases where the moon is aligned just right, the
rectangle would graze over the borders, and so it would take priority
over other borders and show a pointer cursor instead of a resize
cursor.