Previously shutter was instantly opening, staying opened for the shutter time
period of time and then instantly closing. This isn't quite how real cameras
are working, where shutter is opening with some curve. Now it is possible to
define user curve for how much shutter is opened across the sampling period
This could be used for example to make motion blur trails softer.
Shutter curve now can be controlled using curve mapping widget in the motion
blur panel in Render buttons. Only mapping from 0..1 by x axis are allowed,
Y values will be normalized to fill in 0..1 space as well automatically.
Y values of 0 means fully closed shutter, Y values of 1 means fully opened
Default mapping is set to old behavior when shutter opens and closes instantly.
This shutter mapping curve could easily be used by any other render engine by