#17 Multi-segment motion blur

[Magnus Løfgren]

Currently Pixie only seems to support linear interpolation between motion blur "subframes". It seems to only interpolates between the first subframe and the last subframe in the range, and discards the others inbetween.
Multi-segment motion blur is needed for rotating motion, wich accounts for pretty much all forms of animation.

If an example RIB is needed I might post that later when I have access to it.