Fix frame corruption with one frame transition
Fix frame corruption with one frame transition

As reported here:
https://forum.shotcut.org/t/distorted-transitions/34608

j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt