Add alpha channel support to avformat consumer.
Add alpha channel support to avformat consumer.

This requires that you set the pix_fmt property correctly for the codec:
  vcodec=png pix_fmt=rgba
  vcodec=targa pix_fmt=bgra
  vcodec=qtrle pix_fmt=argb

Sometimes you also need to use mlt_image_format=rgb24a to prevent losing
the alpha channel during image processing.

zzhhui/mlt,anba8005/mlt,xzhavilla/mlt,xzhavilla/mlt,mltframework/mlt,siddharudh/mlt,siddharudh/mlt,mltframework/mlt,xzhavilla/mlt,siddharudh/mlt,siddharudh/mlt,j-b-m/mlt,zzhhui/mlt,j-b-m/mlt,wideioltd/mlt,xzhavilla/mlt,anba8005/mlt,j-b-m/mlt,zzhhui/mlt,mltframework/mlt,xzhavilla/mlt,wideioltd/mlt,wideioltd/mlt,zzhhui/mlt,j-b-m/mlt,j-b-m/mlt,wideioltd/mlt,wideioltd/mlt,anba8005/mlt,j-b-m/mlt,xzhavilla/mlt,zzhhui/mlt,j-b-m/mlt,mltframework/mlt,zzhhui/mlt,xzhavilla/mlt,mltframework/mlt,siddharudh/mlt,wideioltd/mlt,anba8005/mlt,xzhavilla/mlt,j-b-m/mlt,siddharudh/mlt,zzhhui/mlt,wideioltd/mlt,mltframework/mlt,anba8005/mlt,anba8005/mlt,mltframework/mlt,siddharudh/mlt,anba8005/mlt,j-b-m/mlt,zzhhui/mlt,mltframework/mlt,mltframework/mlt,zzhhui/mlt,wideioltd/mlt,anba8005/mlt,siddharudh/mlt,mltframework/mlt,xzhavilla/mlt,j-b-m/mlt,anba8005/mlt,wideioltd/mlt,siddharudh/mlt