...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.
title_height is a derived variable which needs both config and theme
variables (font height and title padding). The code is tidier calling
post_processing() for this from theme_init()
