vt: support Unicode recomposition

Try replacing any decomposed Unicode sequence by the corresponding
recomposed code point. Code point to glyph correspondance works best
after recomposition, and this apply mostly to single-width code points
therefore we can't preserve them in their decomposed form anyway.

With all the infrastructure in place this is now trivial to do.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://lore.kernel.org/r/20250410011839.64418-9-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -2953,8 +2953,15 @@ static int vc_con_write_normal(struct vc_data *vc, int tc, int c,
 				 * double-width.
 				 */
 			} else {
-				/* Otherwise zero-width code points are ignored */
-				goto out;
+				/* try recomposition */
+				prev_c = ucs_recompose(prev_c, c);
+				if (prev_c != 0) {
+					vc_con_rewind(vc);
+					c = prev_c;
+				} else {
+					/* Otherwise zero-width code points are ignored */
+					goto out;
+				}
 			}
 		}
 	}