display: avoid determining twice from and until where to draw each row

The two calls of draw_row() are each immediately preceded by a call to
display_string(), which has already determined from which x position
and until which x position in the relevant line the current row will
be drawn -- doing this again in draw_row() is a waste of time.  Even
though it is ugly, pass the two data points from one function to the
other via global variables.

For normal files (without overlong lines), this saves on average some
fifty calls of advance_over() per row.  When softwrapping a file with
overlong lines, the savings for each softwrapped chunk are much higher.

src/winio.c

@@ -54,6 +54,10 @@ static bool linger_after_escape = FALSE;
 		/* Whether to give ncurses some time to get the next code. */
 static int statusblank = 0;
 		/* The number of keystrokes left before we blank the status bar. */
+size_t from_x = 0;
+		/* From where in the relevant line the current row is drawn. */
+size_t till_x = 0;
+		/* Until where in the relevant line the current row is drawn. */
 static bool has_more = FALSE;
 		/* Whether the current line has more text after the displayed part. */
 static bool is_shorter = TRUE;
@@ -1706,6 +1710,8 @@ void set_blankdelay_to_one(void)
 char *display_string(const char *text, size_t column, size_t span,
 						bool isdata, bool isprompt)
 {
+	const char *origin = text;
+		/* The beginning of the text, to later determine the covered part. */
 	size_t start_index = actual_x(text, column);
 		/* The index of the first character that the caller wishes to show. */
 	size_t start_col = wideness(text, start_index);
@@ -1875,6 +1881,10 @@ char *display_string(const char *text, size_t column, size_t span,
 	/* Null-terminate the converted string. */
 	converted[index] = '\0';
 
+	/* Remember what part of the original text is covered by converted. */
+	from_x = start_index;
+	till_x = text - origin;
+
 	return converted;
 }
 
@@ -2437,15 +2447,6 @@ void place_the_cursor(void)
  * from_col is the column number of the first character of this "page". */
 void draw_row(int row, const char *converted, linestruct *line, size_t from_col)
 {
-#if !defined(NANO_TINY) || defined(ENABLE_COLOR)
-	size_t from_x = actual_x(line->data, from_col);
-		/* The position in the line's data of the leftmost character
-		 * that is at least partially onscreen. */
-	size_t till_x = actual_x(line->data, from_col + editwincols - 1) + 1;
-		/* The position in the line's data just after the start of
-		 * the last character that is at least partially onscreen. */
-#endif
-
 #ifdef ENABLE_LINENUMBERS
 	/* If line numbering is switched on, put a line number in front of
 	 * the text -- but only for the parts that are not softwrapped. */