- Using the ON_TOP flag for the overlay shells stops the title bar
  from flashing during the drag.
- Parenting the Tracker under a shell rather than under the display
  prevents the drag operation from randomly failing to respond to
  mouse input.
- Expand the Region for the overlay shells to fill the parent shell,
  so that when the drop shadow appears it gets hidden by the drop
  shadow of the parent window.
- Copy all Rectangles before passing them into Tracker, since Tracker
  modifies the Rectangles it receives.

Change-Id: I88cc62a40069315cc12c5a422373e219c31b0553
Signed-off-by: Stefan Xenos <sxenos@gmail.com>