Created attachment 154996 [details]
Changing Transform's blending modes on the top clip has no effect

SUMMARY
I often need to use the "Screen" or "Multiply" blending modes but changing a Transform Effect's "Composite" (blending mode) has no effect. So I always have to add a composite clip between the clips, choose the "Cairo Affine Blend" and then select "Screen" or other option within the Cairo Affine Blend.

This has been happening since I started using Kdenlive. The blending modes in the Transform effect never worked, but it gets annoying having to add the extra "Composite" clip just to change the blending mode.

STEPS TO REPRODUCE
1. Add two different clips to the timeline, each on a different track, one above the other
2. Add a "Transform" effect to the clip on top
3. Select the clip on top and change the "Composite" (blending mode) property of the "Transform" effect you just added, from "Alpha blend" (the default) to any other option — Screen, Multiply, Overlay, as examples
4. See how the blending mode of the video on top does not change at all

OBSERVED RESULT
Changing a Transform effect's "Composite" (blending mode) has no effect.

EXPECTED RESULT
Changing a Transform effect's "Composite" (blending mode) should change the blending mode of that clip / how that  clip blends with everything that's under it. Basically the "Composite" (blending mode) in the Transform effect should work like the blending modes available in the "Cairo Affine Blend" option in the additional "Composite" clips.

We shouldn't have to go through the extra hurdles of using and managing the additional "Composite" clip.

SOFTWARE/OS VERSIONS
Linux: Pop!_OS 21.10 (Gnome 40.4.0 under X11)
KDE Frameworks Version: 5.100.0
Qt Version: 5.15.7

ADDITIONAL INFORMATION