Add support for avdevice video channel selection.
Add support for avdevice video channel selection.

For example, video4linux2:/dev/video0?channel=2 sets the input to S-

gmarco/mlt-orig,ttill/MLT-roto,ttill/MLT-roto,ttill/MLT,siddharudh/mlt,anba8005/mlt,ttill/MLT-roto,gmarco/mlt-orig,mltframework/mlt,ttill/MLT,ttill/MLT,anba8005/mlt,ttill/MLT,zzhhui/mlt,j-b-m/mlt,gmarco/mlt-orig,ttill/MLT,siddharudh/mlt,siddharudh/mlt,wideioltd/mlt,mltframework/mlt,wideioltd/mlt,wideioltd/mlt,zzhhui/mlt,zzhhui/mlt,j-b-m/mlt,siddharudh/mlt,j-b-m/mlt,zzhhui/mlt,siddharudh/mlt,mltframework/mlt,ttill/MLT-roto,anba8005/mlt,anba8005/mlt,ttill/MLT,j-b-m/mlt,ttill/MLT-roto,ttill/MLT-roto-tracking,siddharudh/mlt,j-b-m/mlt,anba8005/mlt,gmarco/mlt-orig,wideioltd/mlt,mltframework/mlt,wideioltd/mlt,ttill/MLT-roto,xzhavilla/mlt,anba8005/mlt,ttill/MLT,anba8005/mlt,ttill/MLT,xzhavilla/mlt,ttill/MLT-roto-tracking,j-b-m/mlt,gmarco/mlt-orig,xzhavilla/mlt,ttill/MLT-roto-tracking,wideioltd/mlt,ttill/MLT,ttill/MLT-roto,zzhhui/mlt,ttill/MLT-roto-tracking,wideioltd/mlt,zzhhui/mlt,j-b-m/mlt,mltframework/mlt,gmarco/mlt-orig,xzhavilla/mlt,gmarco/mlt-orig,xzhavilla/mlt,gmarco/mlt-orig,mltframework/mlt,mltframework/mlt,ttill/MLT-roto-tracking,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,wideioltd/mlt,ttill/MLT-roto,xzhavilla/mlt,siddharudh/mlt,mltframework/mlt,ttill/MLT-roto-tracking,ttill/MLT-roto-tracking,xzhavilla/mlt,siddharudh/mlt,siddharudh/mlt,zzhhui/mlt,wideioltd/mlt,anba8005/mlt,gmarco/mlt-orig,xzhavilla/mlt,anba8005/mlt,zzhhui/mlt,ttill/MLT-roto,ttill/MLT-roto-tracking,zzhhui/mlt,xzhavilla/mlt,j-b-m/mlt,ttill/MLT-roto-tracking