patch 9.2.0819: MS-Windows: sixel image shown as raw text in the console

patch 9.2.0819: MS-Windows: sixel image shown as raw text in the console

Problem:  On the MS-Windows console a popup image is displayed as the raw
          sixel escape sequence instead of the image, unless
          'termguicolors' is set (Bakudankun).
Solution: Write a DCS sequence with the console API that lets the console
          parse it, no matter which write path the current colors select.
          Keep track of the terminator across writes, since a long
          sequence is split up (Hirohito Higashi).

fixes:  #20795
closes: #20797

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>