display is where pixels are drawn to and user input are read from 


struct mbtk_display_s {
  union { void *ptr[64]; long int i[64]; } backend_data; /* opaque backend data */
  mbtk_backend_t *be;

  mbtk_window_t *topwin;
  gdl_list_t windows;          /* all windows open within this display */
  unsigned need_win_redraw:1;  /* if any of the windows needs a window redraw */
  double win_next_redraw;      /* when to next redraw windows (0 means default timeout) */

  const mbtk_style_t *style;   /* style implementation to use for drawing widgets that don't have their own style */
  mbtk_styleprop_t *styleprop; /* fallback (default) style properties for all widgets without own styleprop */

  unsigned short button_mask; /* mouse button mask: each bit is the last known state of a mouse button */

  struct {
    mbtk_event_t ev_win_resize, ev_win_expose;
    double exp_win_resize, exp_win_expose; /* when ev_win_* will next expire */
    double last_win_resize, last_win_expose; /* when ev_win_* was last done */
  } evi; /* event handler internals */

  mbtk_font_id_t default_fid; /* labels are drawn with this font by default */
  htip_t mpm; /* mpm; key is "name" (mbtk_kw_t), value is (mbtk_mbm_styled_t *); see mbm.h */

  gdl_elem_t link;    /* all displays within a backend */
  vtp0_t timers;      /* all timers that are still allocated */
  void *timerq;       /* timer queue for currently running timers */

#if 0
  /* last popup is the topmost */
  int *popups;
  int popup_used, popup_alloced;
};

/* Change the top window of display. By default the first window created
   under a display becomes the top window. Top window is used when reporting
   generic, window-idependent events (such as timers, fdwatch, stdin, etc.) */
void mbtk_display_set_topwin(mbtk_display_t *disp, mbtk_window_t *win);

/* Query backend and return bitfield of keyboard modifiers currently pressed */
mbtk_keymod_t mbtk_display_query_kbd_mods(mbtk_display_t *disp);