Using "Key Alpha" didn't work when using MBlur render. The accumulation
code was assuming regular alpha then. Now it corrects for it.
Still it's a bit of a weak spot in Blender's render system. I will look
in the future to make this a real post process; converting all RGBA
buffers in the system, including for all passes, to "key alpha".
Combined with that our compositor should become alpha type aware too.
Everything in Blender assumes premul alpha, which still just will work
best in general...
c09b1a985c