[Cin] Multitrack performance issues

[Stefan de Konink]

I think have a significant opportunity to improve the Cinerella 
performance, under normal editing conditions, by n_videotracks-fold. At 
this point in time I notice that the CPU usage (decoding) for a section 
where multiple tracks are stacked, but only the top one is actually on 
screen (read opacity 100%) could be improved.

I observe the following:

  - for a disabled video effect, the CPU still runs
  - for stacked video, the CPU still runs for both videos
  - the effect must be detached and the video below must have play track
    disabled to prevent the CPU to go from 20% to 150%
  - this is independent of colourspace (RGB, RGBA, YUV or YUVA)

-- 
Stefan
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