display: for a large paste or insertion, recalculate the multiline cache

When a large piece of text or code is pasted or inserted, it could
contain matches for start= and end= regexes, and backtracking from
the current screen could mistake an end for a start and could thus
miscolor things.  Avoid this by recalculating the multiline cache
for pastes and insertions that cover more than a screenful.

This fixes https://savannah.gnu.org/bugs/?59982.

Bug existed since version 2.6.0, but existed also before 2.4.3.

src/color.c

@@ -289,8 +289,7 @@ void precalc_multicolorinfo(void)
 	regmatch_t startmatch, endmatch;
 	linestruct *line, *tailline;
 
-	if (openfile->syntax == NULL || openfile->syntax->nmultis == 0 ||
-					openfile->filetop->multidata || ISSET(NO_SYNTAX))
+	if (!openfile->syntax || openfile->syntax->nmultis == 0 || ISSET(NO_SYNTAX))
 		return;
 
 //#define TIMEPRECALC  123

src/cut.c

@@ -734,6 +734,10 @@ void paste_text(void)
 	/* If we pasted less than a screenful, don't center the cursor. */
 	if (less_than_a_screenful(was_lineno, was_leftedge))
 		focusing = FALSE;
+#ifdef ENABLE_COLOR
+	else
+		precalc_multicolorinfo();
+#endif
 
 	/* Set the desired x position to where the pasted text ends. */
 	openfile->placewewant = xplustabs();

src/files.c

@@ -498,6 +498,7 @@ void prepare_for_display(void)
 
 #ifdef ENABLE_COLOR
 	/* Precalculate the data for any multiline coloring regexes. */
+	if (!openfile->filetop->multidata)
 		precalc_multicolorinfo();
 	have_palette = FALSE;
 #endif
@@ -818,6 +819,10 @@ void read_file(FILE *f, int fd, const char *filename, bool undoable)
 	/* If we inserted less than a screenful, don't center the cursor. */
 	if (undoable && less_than_a_screenful(was_lineno, was_leftedge))
 		focusing = FALSE;
+#ifdef ENABLE_COLOR
+	else if (undoable)
+		precalc_multicolorinfo();
+#endif
 
 #ifndef NANO_TINY
 	if (undoable)