Restore src alpha on "burned" image.
Restore src alpha on "burned" image.

The filter reset all alpha values to 0. This isn't a problem if sent directly
to some consumer who ignore alpha values. But when passed to a transition as a
"b" frame, the content would be completely transparent, thus showing only the
"a" frame.

Test case:
melt -video-track color:red -video-track frei0r.test_pat_G -filter burningtv
-transition composite fill=1

Before this change, the output would be red frame. After this fix, the output
is the pattern with fire on the squares - and the red frame completly obscured.

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