Avoid Memory copy from prediction result returned by GstBackend

[josejich]

A memory copy is added on GstBackend from the prediction result after commit 88d0cd01ab4b67f15d7fca07dfc42c892c6463d1 in order to return a new allocated gpointer not associated with the IPrediction. No performance hits are taken on the tests perform yet since a typical prediction is not of large size, yet the copy could be avoided if the 'IPrediction stayed alive to perform the post-process call.