kdenlive render pipeline (was Re: smart encoding)

[Mikko Rapeli]

Related to this, kdenlive render pipeline for time line preview on
the monitor GUI component is far from optimal. First the clips are rendered
to project profile and then rescaled to fit the monitor screen size.
Also with proxy clips.

IMO better would be to render project previews directly to the
current monitor/preview size without passing through current project
profile except for frame rate.

For final rendering, naturally, the project profile should be used.

https://bugs.kde.org/show_bug.cgi?id=384764

-Mikko