...which is used by desktop_view_at() and ssd_at() to determine which
decoration part is under the cursor.
ssd_interactive_box() allows deocoration parts to extend outside the
visible regions, and therefore supports a wider 'resize-edges' area than
the border itself.
It makes more sense to just keep one set of corner textures for server
side view decorations, rather than storing a set for each view. This also
keeps the code simpler when when changing theme parameters.
