patch 9.1.0106: Visual highlight hard to read with 'termguicolors'

Problem:  Visual highlight hard to read with 'termguicolors'
          (Maxim Kim)
Solution: Set Visual GUI foreground to black (with background=light)
          and lightgrey (with background=dark)
          (Maxim Kim)

fixes: #14024
closes: #14025

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/highlight.c b/src/highlight.c
index 7fbeeb7..9b3b072 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -222,7 +222,7 @@
 	 "SignColumn term=standout ctermbg=Grey ctermfg=DarkBlue guibg=Grey guifg=DarkBlue"),
 #endif
     CENT("Visual ctermbg=Grey ctermfg=Black",
-	 "Visual ctermbg=Grey ctermfg=Black guibg=LightGrey"),
+	 "Visual ctermbg=Grey ctermfg=Black guibg=LightGrey guifg=Black"),
 #ifdef FEAT_DIFF
     CENT("DiffAdd term=bold ctermbg=LightBlue",
 	 "DiffAdd term=bold ctermbg=LightBlue guibg=LightBlue"),
@@ -311,7 +311,7 @@
 	 "SignColumn term=standout ctermbg=DarkGrey ctermfg=Cyan guibg=Grey guifg=Cyan"),
 #endif
     CENT("Visual ctermbg=Grey ctermfg=Black",
-	 "Visual ctermbg=Grey ctermfg=Black guibg=#575757"),
+	 "Visual ctermbg=Grey ctermfg=Black guibg=#575757 guifg=LightGrey"),
 #ifdef FEAT_DIFF
     CENT("DiffAdd term=bold ctermbg=DarkBlue",
 	 "DiffAdd term=bold ctermbg=DarkBlue guibg=DarkBlue"),