Bug 369180

When I've preview rendered a zone, and make changes to an Effect parameter on a clip IN that zone, the preview render bar shows red where it should. However, when I click on Preview Render, it instantly goes green and uses the cached render files -- it doesn't re-render the clip with the changed effects that I made.

Reproducible: Always

Steps to Reproduce:
1. Preview render a zone in timeline with clips with effects on them.
2. Change some of the parameters to those clips.
3. Re-render same preview zone.

Actual Results:  
Kdenlive uses the already rendered cache files, instead of re-rendering the video clips whose effects I made changes to. I can manually delete the cache, but then I have to re-render everything.

Expected Results:  
When changes are made to an effect on a clip in the Timeline, Kdenlive recognizes it and puts a red bar in the space where a new preview render is needed (which it does). BUT, when I hit Start preview render, I would expect it to re-render the clips that have had their effects parameters changed.

Bug discovered while using Kdenlive 16.11.70 git master build via ppa:kdenlive/kdenlive-master Kubuntu 16.10, KDE Plasma 5.7.2 desktop environment. 

KDE Frameworks 5.24.0
Qt 5.6.1