Add origin-shifting (Shift+cursors) and source-shifting (Ctrl+cursors) to Net.

(Adding modifier+cursors handling has had minor knock-on effects on the other
puzzles, so that they can continue to ignore modifiers.)

(An unfortunate side effect of this is some artifacts in exterior barrier
drawing; notably, a disconnected corner can now appear at the corner of the
grid under some circumstances. I haven't found a satisfactory way round
this yet.)

[originally from svn r5844]

cube.c

@@ -1013,6 +1013,8 @@ static game_state *make_move(game_state *from, game_ui *ui,
     int i, j, dest, mask;
     struct solid *poly;
 
+    button = button & (~MOD_MASK | MOD_NUM_KEYPAD);
+
     /*
      * All moves are made with the cursor keys or numeric keypad.
      */