[3.0] Fix for FrameBufferImage.

[3.0] Fix for FrameBufferImage.

* The Connect() and Disconnect() methods are unnecessarily
  implementing a counter that tracks how many handlers are
  observing the frame buffer image. In case that the frame
  buffer image is not observed by any handler, the internal
  FBO is destroyed. A handler may observe again the frame
  buffer image but the FBO has already been destroyed.

  This caused an issue with the GaussianBlurView effect which
  was using a render task to blur an image and a frabe buffer
  image to write the blurred image.

Change-Id: I89a7de6f97ee52fb219f8b7641d4d2bbb42764d5
Signed-off-by: Victor Cebollada <v.cebollada@samsung.com>