Same calculation as for RGBA etc to create unpremultiplied output.
Useful for creating an intermediate "Mix/Effects bank", in vision mixer
(switcher) terminology.

Example command:

ffmpeg -i input.mov -vf
"[in]scale=iw:ih:interl=1,format=yuv420p[bg];movie=BBC_blocks_watermark.png,scale=iw:ih:interl=1,format=yuva420p[dog];movie=Kickabout_strap.mov,scale=iw:ih:interl=1,format=yuva420p[strap];[strap][dog]overlay=0:0[me1];[bg][me1]overlay=0:0,scale=0:0:interl=1[out]"
-b:v 3M -an output.mov

Fix trac ticket #549.
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>