Add 5.1 to stereo downmix to audiochannels filter.
Add 5.1 to stereo downmix to audiochannels filter.

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