Interpret Meta as Alt

Shift+Alt often generates Meta on Unix systems. Assume this is the
same thing as Alt on Windows.
diff --git a/win/rfb_win32/keymap.h b/win/rfb_win32/keymap.h
index 664312a..b9b6c31 100644
--- a/win/rfb_win32/keymap.h
+++ b/win/rfb_win32/keymap.h
@@ -134,6 +134,8 @@
   { XK_Control_R,        VK_CONTROL, 1 },
   { XK_Alt_L,            VK_MENU, 0 },
   { XK_Alt_R,            VK_MENU, 1 },
+  { XK_Meta_L,           VK_MENU, 0 },
+  { XK_Meta_R,           VK_MENU, 1 },
 
   // Left & Right Windows keys & Windows Menu Key