Optimize media rendering during slideshow: save one grabFrame() call

'FallbackGraphic' property introduced in:
b862a216f769d10e726ad759762b5e6a412acdaf
contains the previously called grabFrame's
result, so we can use that instead of render the media
frame again (it affects glTF models too).

Change-Id: Ia93e771e92cc7630480c665dea66fbf571dfa2a0