[kdenlive] Track compositing changes

[Jean-Baptiste Mardelle]

Hi everyone,

I just commited a change to MLT and Kdenlive that enable a new transition 
and a new effect for compositing and transform.

I know it is a bit late to introduce such change so close to the 16.08 
release but it should fix a major slowdown issue we were experiencing since 
the introduction of the track compositing feature (1). This should also 
enable us (with another pending patch) to fix the titles being pixelated in 
some situations (2).

This change replaces the frei0r.cairoblend transition that was used to 
create transparency between tracks with a new transition called qtblend. 
This is currently only enabled on newly created projects, when you have the 
latest MLT git master.

This new transition should bring a great speed improvement in situations 
where several non transparent clips were stacked on different tracks.

The new qtblend filter, called "Transform" in effect stack brings the same 
features as the affine (position and zooom) filter, but has a better 
handling of alpha transparency and supports the recent animated keyframe 
feature allowing curved moves.

Feedback and testing is welcome.
For *ubuntu users, our git master PPA (if you want to take the risk) should 
be updated in a few hours with the changes.

Regards,

jb

(1) https://bugs.kde.org/show_bug.cgi?id=361360
(2) https://bugs.kde.org/show_bug.cgi?id=364891
(3) https://launchpad.net/~kdenlive/+archive/ubuntu/kdenlive-master