Transparency problem with qtblend/timeline transparent tracks

[Harald Albrecht]

Hi,

is there some issue with the transparency detection in recent qtblend? 
I've a png clip on the lowest track and then put a title on the next 
higher track. The title is "special" in that it has transparency, but 
only in some part. In particular, there is no transparency in the upper 
part of the title clip (a big gray bar at the top from 0,0 to 1920,100). 
The result is that only the title clip is shown, but the lower track 
does not shine through the transparent title clip parts.

When I'm using composite or affine, or setting timeline to "preview" 
then track compositing works as expected.

I'm on Kubuntu 16.10, with Kdenlive master ppa enabled. libmlt is 
6.3.0+git201610231655~ubuntu16.10.1.

Best regards,
Harald