This adds a per preview option to set render sizes (which proxy size is used).
That makes it possible to have
* several small preview screens which update really fast using the proxy files
* one large output screen that operates in full resolution
Since most of the time not all input strips are considered when calculating
an output screen, this is a big win.
Also: one can disable a preview screen completely using this option.
Other use cases: vector + chromaviews don't always need full resolution
pictures and work equally well on proxy files!
This option finally makes my working setup _completely_ realtime :)
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