...one image onto another

gimp_dock_window_display_changed(): make sure the "auto-follow-active"
logic works both ways: when the active image or display is changed in
a dockable, update the global context. Fixes multi-window mode.

gimp_context_real_set_display(): make sure a context's display and
image are always in a consistent state and never have a display that
is not display->image: when display is the same as context->display,
check that the context's image matches display->image, so that after a
gimp_context_set_display(), the context is consistent in all
cases. Fixes single-window mode.