smart rendering mode is broken

Submitted by Sebastian Dröge `@slomo`

It's currently broken on multiple layers:

there is no option in ges-launch, that's trivial though ;)
set_render_settings() will create the pads on encodebin and allows to set the output filename, set_mode() allows to set the avoid-reencoding property on encodebin. The property must be set before the pads are created, but the output filename must be set before set_mode() is called with the SMART_RENDER mode. I propose to make the output filename only required once the pipeline goes to READY, and not check for that before.
in the lower layers things go wrong when working around all the above, I didn't investigate further. The pipeline got stuck.