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]

netslide.c

@@ -1006,6 +1006,8 @@ static game_state *make_move(game_state *state, game_ui *ui,
     int n, dx, dy;
     game_state *ret;
 
+    button &= ~MOD_MASK;
+
     if (button != LEFT_BUTTON && button != RIGHT_BUTTON)
         return NULL;