Move display_plot* over to the UIs and remove the screen copy in display.c
to allow SDL (and maybe GTK if Phil is so inclined) to draw directly into
their existing temporary screens ready for scaling to reduce unnecessary
copying of screen data. Restore display_last_screen for less overdraw in
screen/mode switch situations. Move border updating into the
display_*_dirty/display_plot8 system to reduce overdraw with frequent border
changes - e.g. loading and Shock II. Fix updating of last border line after
the end of the screen (fixes Shock II last border line). Bulk allocate
border change objects. Make display_get_addr a macro. (patch #100).
Legacy-ID: 2658
