Don't advance the main RNG during hallucination

This is based on the multiple-RNGs code fron NetHack4, but using
only the parts relevant to the display RNG (and with substantial
changes, both because of post-3.4.3 changes, and because Nethack4's
display code is based on Slash'EM's rather than NetHack's).

src/mthrowu.c

@@ -532,7 +532,7 @@ struct obj *obj;         /* missile (or stack providing it) */
      * be careful not to use either one after it's been freed.
      */
     if (sym)
-        tmp_at(DISP_FLASH, obj_to_glyph(singleobj));
+        tmp_at(DISP_FLASH, obj_to_glyph(singleobj, rn2_on_display_rng));
     while (range-- > 0) { /* Actually the loop is always exited by break */
         bhitpos.x += dx;
         bhitpos.y += dy;