Cycles: Expose user-defined shutter curve to the interface

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
shutter.

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
accessing scene.render.motion_blur_shutter_curve.

Reviewers: #cycles, brecht, juicyfruit, campbellbarton

Differential Revision: https://developer.blender.org/D1585

source/blender/blenloader/intern/readfile.c

@@ -6007,6 +6007,8 @@ static void direct_link_scene(FileData *fd, Scene *sce)
 	}
 
 	sce->preview = direct_link_preview_image(fd, sce->preview);
+
+	direct_link_curvemapping(fd, &sce->r.mblur_shutter_curve);
 }
 
 /* ************ READ WM ***************** */