[Problem] Bubble emitter's background render task is deleted on completion.
Context loss removes frame buffer image, and there is no mechanism to regenerate it.
[Cause] New requirement
[Solution] Add listener to Stage::ContextRegainedSignal() to re-run the
render task and regenerate the background image.
Change-Id: I08b54b286b04d22b923785b0634497f13380045f
Signed-off-by: David Steele <david.steele@partner.samsung.com>