mlterm: update to 3.8.4.

Changes noted in doc/en/RelaseNote:

ver 3.8.4
* Support DECSLPP, DECSCPP, DECBBSM, DECRQTSR, DECCTR, DECRQPSR, DECSNLS, DECSTGLT, DECRSPS,
  DECSR, CSI 7 t, CSI 9;1 t, CSI ? Pm s, CSI ? Pm r, CSI 34 [hl], CSI 53 m and CSI 55 m.
* Support DECSASD, DECSSDT unless vertical mode is enabled.
* Support 90-degree rotation of alphabet glyphs in vertical mode (cjk alone) on framebuffer,
  wayland and android if freetype is enabled.
* Support sticky shift key of skk im module, and change the format of skk option from
  "skk:[dictionary]" to "skk:dict=[dictionary],sskey=[sticky shift key]".
* Add "use_vertical_cursor" and "click_interval" to OSC 5379, 5380 and 5381.
* Add "ot_features" and "ot_script" to OSC 5380 and 5381.
* Add "Features" and "Script" buttons to "Encoding" tab of mlconfig.
* Add "Underline pos (pixels)" and "Baseline pos (pixel)" to "Font" tab of mlconfig.
* Add "Double click interval (msec)", "Send keys to all windows" and
  "Trim trailing CR/LF in pasting" to "Others" tab of mlconfig.
* Add doc/en/ControlSequences which lists escape/control sequences which mlterm supports.
* Change the default value of "use_vertical_cursor" option from false to true.
* Drop "CSI ? 9500 [hl]".
* Improve processing sixel graphics on pseudo color framebuffer.
* Merge patches:
  https://sf.net/p/mlterm/patches/_discuss/thread/f5dc445d/0fc3/attachment/fix_spelling.diff
* Bug fixes:
  Fix impossibility of keyborad and pointer input after switching virtual consoles on wayland.
  Fix unexpectedly having privilege in opening a shell on startup of mlterm built with brlapi.
  Fix Pl, Pc and Pp of XCPR and CPR.
  Fix corruption of TBC (CSI g), CTC (CSI W) and DECATC(CSI ,}).

(tsutsui)