Fix left column flicker on sliced pixfmt conversion.
Fix left column flicker on sliced pixfmt conversion.

Fixes Shotcut bug https://github.com/mltframework/shotcut/issues/418

Require the width of the last slice to be a multiple of 8.

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