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]

puzzles.h

@@ -32,7 +32,10 @@ enum {
     CURSOR_LEFT,
     CURSOR_RIGHT,
     
-    MOD_NUM_KEYPAD = 0x40000000
+    MOD_CTRL       = 0x10000000,
+    MOD_SHFT       = 0x20000000,
+    MOD_NUM_KEYPAD = 0x40000000,
+    MOD_MASK       = 0x70000000 /* mask for all modifiers */
 };
 
 #define IS_MOUSE_DOWN(m) ( (unsigned)((m) - LEFT_BUTTON) <= \