When the background had an alpha channel, the old code in blend_plane
calculated premultiplied alpha from the destination plane colors instead of the
destination alpha.

Also the calculation of the output alpha should only happen after the color
planes are already finished.

Fixes output of:
ffplay -f lavfi "testsrc2=alpha=32[a];color=black[b];[b][a]overlay[out0]"
Signed-off-by: Marton Balint <cus@passwd.hu>