DSRenderEngineDaliImpl-test: Add a new testcase for compositing twe windows.

DSRenderEngineDaliImpl-test: Add a new testcase for compositing twe windows.

 - This test shows a simple compositing of two windows.
 - The first added windowA was created in 500x500 size and red color.
 - The second added windowB was created in 250x250 size and blue color.
 - Since Dali's scene graph traverse method is DFS,
  windowA added first is rendered first,
  and windowB is rendered on top of it.

   ┌─────┬─────┐
   │  B  │     │
   │ blue│  A  │
   ┣─────┘     │
   │    red    │
   │    　     │
   └───────────┘

Change-Id: I7b0807ebd5e9c4c41983f7830f5041dad700b3d6
Signed-off-by: Joonbum Ko <joonbum.ko@samsung.com>