preload regression fix
Created by: nicosang
Hi,
this PR has to solve a regression of preload mode. The main issue was that getBuffer function was not defined in PreBufferSink class. The second one was that the video StreamProcessor wasn't created because video element is undefined in this use case.
So, it solves issue #2984 (closed). @bknill, could you, please, take a look at it?
Nico