Misc #204: Add glx_take_screenshot() & others

- Add glx_take_screenshot() for taking a screenshot. With ImageMagick
  the output data could be viewed in this way:

    display -size [SCREEN-WIDTH]x[SCREEN-HEIGHT] -depth 8 -flip
    rgb:[PATH]

  (#204)

- Add D-Bus command `opts_get string:paint_on_overlay_id` to get X
  Composite overlay window ID. (#204)

- Code cleanup.

src/dbus.c

@@ -901,6 +901,11 @@ cdbus_process_opts_get(session_t *ps, DBusMessage *msg) {
   cdbus_m_opts_get_do(fork_after_register, cdbus_reply_bool);
   cdbus_m_opts_get_do(detect_rounded_corners, cdbus_reply_bool);
   cdbus_m_opts_get_do(paint_on_overlay, cdbus_reply_bool);
+  // paint_on_overlay_id: Get ID of the X composite overlay window
+  if (!strcmp("paint_on_overlay_id", target)) {
+    cdbus_reply_uint32(ps, msg, ps->overlay);
+    return true;
+  }
   cdbus_m_opts_get_do(unredir_if_possible, cdbus_reply_bool);
   cdbus_m_opts_get_do(unredir_if_possible_delay, cdbus_reply_int32);
   cdbus_m_opts_get_do(redirected_force, cdbus_reply_enum);