plugins/shakecursor: Display default cursor shape

At the moment, the shake cursor respects the current cursor shape. But there are couple of drawbacks behind doing it: the first is that it's very likely that the cursor pixmap has low resolution, the second is the cursor can be hidden client-side.

This change makes the shake cursor plugin load the Xcursor theme with a high enough size and display the default cursor shape regardless of what the client has set in order to address the two aforementioned issues.