WindowServer: Add a mode for running without buffer flipping

We can't rely on all hardware to give us a way to flip between the back
and front buffer.

This mode should actually perform slightly better but may show some tearing
as we don't have a way to know when we're in vertical retrace.

Servers/WindowServer/WSCompositor.h

@@ -35,6 +35,8 @@ public:
     void invalidate_cursor();
     Rect current_cursor_rect() const;
 
+    bool can_flip_buffers() const { return true; }
+
 private:
     WSCompositor();
     void flip_buffers();