Bug#993832: synfig FTBFS with mlt 7

[peter green]

Tags 993832 +patch
thanks

I found a fix in the upstream git repo and was able to apply it to the Debian
package and build sucessfully in raspbian bookworm-staging. A debdiff is
available at 
https://debdiffs.raspbian.org/main/s/synfig/synfig_1.4.0+dfsg-2+rpi1.debdiff
no intent to NMU in Debian.