patch 9.2.0216: MS-Windows: Rendering artifacts with DirectX

patch 9.2.0216: MS-Windows: Rendering artifacts with DirectX

Problem:  MS-Windows: Rendering artifacts with DirectX
          (Alexander Zhura)
Solution: Force redraw (Yasuhiro Matsumoto)

DirectWrite subpixel rendering (especially with CFF/OTF fonts) can
extend glyph pixels beyond cell boundaries.  Vim already handles the
forward direction (redraw the next character when the current one
changes) for MS-Windows antialiasing, but the backward direction was
missing.

Add gui.directx_enabled flag accessible from screen.c and extend the
existing spill-over handling to:
- Redraw the current character when the previous one changed (backward)
- Force redraw of the next character in screen_puts_len() and
  screen_fill() paths

fixes:  #19586
closes: #19761

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>