fix regression in 2c354d6e with multiple affine filters
fix regression in 2c354d6e with multiple affine filters

See #9 in https://forum.shotcut.org/t/v20-09-beta-is-now-available-to-
test/20766/31

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