fix stb_textedit redo, from omar cornut

stb_textedit.h

@@ -29,7 +29,7 @@
 //
 // VERSION HISTORY
 //
-//   1.12 (2018-01-29) user can change STB_TEXTEDIT_KEYTYPE
+//   1.12 (2018-01-29) user can change STB_TEXTEDIT_KEYTYPE, fix redo to avoid crash
 //   1.11 (2017-03-03) fix HOME on last line, dragging off single-line textfield
 //   1.10 (2016-10-25) supress warnings about casting away const with -Wcast-qual
 //   1.9  (2016-08-27) customizable move-by-word
@@ -1119,15 +1119,18 @@ static void stb_textedit_discard_redo(StbUndoState *state)
       // if the k'th undo state has characters, clean those up
       if (state->undo_rec[k].char_storage >= 0) {
          int n = state->undo_rec[k].insert_length, i;
-         // delete n characters from all other records
+         // move the remaining redo character data to the end of the buffer
          state->redo_char_point = state->redo_char_point + (short) n; // vsnet05
-         STB_TEXTEDIT_memmove(state->undo_char + state->redo_char_point, state->undo_char + state->redo_char_point-n, (size_t) ((STB_TEXTEDIT_UNDOSTATECOUNT - state->redo_char_point)*sizeof(STB_TEXTEDIT_CHARTYPE)));
+         STB_TEXTEDIT_memmove(state->undo_char + state->redo_char_point, state->undo_char + state->redo_char_point-n, (size_t) ((STB_TEXTEDIT_UNDOCHARCOUNT - state->redo_char_point)*sizeof(STB_TEXTEDIT_CHARTYPE)));
+         // adjust the position of all the other records to account for above memmove
          for (i=state->redo_point; i < k; ++i)
             if (state->undo_rec[i].char_storage >= 0)
-               state->undo_rec[i].char_storage = state->undo_rec[i].char_storage + (short) n; // vsnet05
+               state->undo_rec[i].char_storage += (short) n; // vsnet05
       }
+      // now move all the redo records towards the end of the buffer; the first one is at 'redo_point'
+      STB_TEXTEDIT_memmove(state->undo_rec + state->redo_point+1, state->undo_rec + state->redo_point, (size_t) ((STB_TEXTEDIT_UNDOSTATECOUNT - state->redo_point)*sizeof(state->undo_rec[0])));
+      // now move redo_point to point to the new one
       ++state->redo_point;
-      STB_TEXTEDIT_memmove(state->undo_rec + state->redo_point-1, state->undo_rec + state->redo_point, (size_t) ((STB_TEXTEDIT_UNDOSTATECOUNT - state->redo_point)*sizeof(state->undo_rec[0])));
    }
 }
 
@@ -1210,11 +1213,11 @@ static void stb_text_undo(STB_TEXTEDIT_STRING *str, STB_TexteditState *state)
 
          // there's definitely room to store the characters eventually
          while (s->undo_char_point + u.delete_length > s->redo_char_point) {
-            // there's currently not enough room, so discard a redo record
-            stb_textedit_discard_redo(s);
             // should never happen:
             if (s->redo_point == STB_TEXTEDIT_UNDOSTATECOUNT)
                return;
+            // there's currently not enough room, so discard a redo record
+            stb_textedit_discard_redo(s);
          }
          r = &s->undo_rec[s->redo_point-1];
 
@@ -1285,6 +1288,7 @@ static void stb_text_redo(STB_TEXTEDIT_STRING *str, STB_TexteditState *state)
    if (r.insert_length) {
       // easy case: need to insert n characters
       STB_TEXTEDIT_INSERTCHARS(str, r.where, &s->undo_char[r.char_storage], r.insert_length);
+      s->redo_char_point += r.insert_length;
    }
 
    state->cursor = r.where + r.insert_length;