[Cin] Multitrack performance issues

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