Create Region on effects is non-working

Hi, Steps to reproduce :

• In Gimp, create a png 1920x1080, with transparent and opaque areas.
• In kdenlive, create a project HD 1080i 25fps
• Add a video clip to the project and to the timeline
• Add a colorize effect to the clip on the timeline
• In the effect menu, choose Create Region and select your png

Expected result :

• The colorize effect applies only on the areas depending on the transparency
• as shown in this video. #399 (closed)

Actual result :

• Nothing changes, the effect still applies to the whole image, the project is not modified.

Version used : refactoring timeline commit 75e552cb Note : there was already a problem in the previous versions as described in https://youtu.be/ysMTH1_hI6w?t=46

• If it helps I think the MLT filter is called region. #399 (closed)
• The effect xml file is in the source code](https://invent.kde.org/kde/kdenlive/blob/master/data/effects/region.xml). #399 (closed)

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See here: https://bugs.kde.org/show_bug.cgi?id=425274