changing level workaround (C341-13)

Use a new flush_screen(-1) call to toggle 3rd screen update in goto_level.
This keeps the 3rd screen state unchanged, no matter what happens at higher
levels, ensures the map window cursor stays on the hero, ensures the
hero's showrace colors remains bright white, and so on.

doc/fixes34.3

@@ -81,6 +81,7 @@ when polymorphed into a quantum mechanic on a no teleport level and swallowed,
 allow Conflict-resistant monsters to respond to conflict attacks rather than
 	sitting there and taking the attacks until they die
 prefer herbivorous stone-to-flesh message when hero is a vegitarian
+try even harder to avoid incorrect map display while changing levels
 
 
 Platform- and/or Interface-Specific Fixes

src/display.c

@@ -1346,8 +1346,11 @@ flush_screen(cursor_on_u)
      *	    flush_screen->print_glyph->impossible->pline->flush_screen
      */
     static   boolean flushing = 0;
+    static   boolean delay_flushing = 0;
     register int x,y;
 
+    if (cursor_on_u == -1) delay_flushing = !delay_flushing;
+    if (delay_flushing) return;
     if (flushing) return;	/* if already flushing then return */
     flushing = 1;
 

src/do.c

@@ -1094,6 +1094,7 @@ boolean at_stairs, falling, portal;
 	/* do this prior to level-change pline messages */
 	vision_reset();		/* clear old level's line-of-sight */
 	vision_full_recalc = 0;	/* don't let that reenable vision yet */
+	flush_screen(-1);	/* ensure all map flushes are postponed */
 
 	if (portal && !In_endgame(&u.uz)) {
 	    /* find the portal on the new level */
@@ -1243,7 +1244,7 @@ boolean at_stairs, falling, portal;
 	/* Reset the screen. */
 	vision_reset();		/* reset the blockages */
 	docrt();		/* does a full vision recalc */
-	flush_screen(1);
+	flush_screen(-1);
 
 	/*
 	 *  Move all plines beyond the screen reset.