Bug#809874: libpng transition started. Raising bug severity.

[Tobias Frost]

Control: Severity -1 serious

The libpng 1.6 transition has now started, therefore this bugs'
severity is now raised to release critical level:
Libpng1.2 will be removed after the transition is completed.

The transition bug is #650601, the transition tracker is 
https://release.debian.org/transitions/html/libpng1.6.html

-- 
tobi