window: Reset tile-mode on maximization

We used to restore side-by-side tiling when unmaximizing, so we
kept the tile-mode during maximization. Since commit 10d53fc7d
there's no longer a good reason to do so, and it can result in
tile previews being shown erroneously on window drag operations
without motion (double-click on titlebar), so reset the tile
mode in maximize().

https://bugzilla.gnome.org/show_bug.cgi?id=682779

src/core/window.c

@@ -3604,6 +3604,7 @@ meta_window_maximize (MetaWindow        *window,
           saved_rect = &window->saved_rect;
 
           window->maximized_vertically = FALSE;
+          window->tile_mode = META_TILE_NONE;
         }
 
       meta_window_maximize_internal (window,