Add 'reverse' parameter to transition 'mix'
Intended to fix this bug: https://bugs.kde.org/show_bug.cgi?id=453537
The video part of the crossfade(not sure if it's the right name) uses the 'luma' MLT transition with its 'reverse' parameter properly set, the audio part also needs to set the 'reverse' parameter, but it's missing in the current version of Kdenlive. Adding this parameter should fix this issue.