Bug 358947

A while ago I asked about the possibility of having multiple processing threads. 
Actually using more than one processing thread is causing all sorts of issues, though, as the filter stack seems to be handled in a weird way. 

It would appear that each of the filters is assigned to a CPU, and spits frames out as fast as it can. This causes mayor effects on the video, as the filter stack is applied out of order. Sometimes some of the filters are not applied to the frames at all. 

It may be possible to fix this by assigning a CPU to each frame, and let the filters be applied in the correct order on a frame by frame basis. 
With an increase in CPU "threads" more frames can be read at the same time. 

The output frames can then be stitched back together in a video for rendering, at least. 
Maybe multiple threads should be disabled for the preview window, as each filter is passed into another filter, and thus cannot be sped up by assigning more CPU.

Reproducible: Always

Steps to Reproduce:
1. Set up kdenlive to use multiple processing threads
2. make a project with complicated filter/effect stacks
3. Render

Actual Results:  
Rendered video is randomly weird, as if some of the filters did not apply, or were applied in the wrong order. 

Expected Results:  
Output video to look exactly like video that was rendered with 1 processing thread.