Cycles: Add support for motion blur position

This adds an option to control at what time relative to the current frame
the shutter is fully opened. Supported options are:

- Shutter is starting to open at the current frame
- Shutter is fully opened at the current frame
- Shutter is fully closed  at the current frame

Custom shutter time offset is possible, same as custom curve for shutter
openness but those are considered nice things to have rather than something
crucial.

Reviewers: juicyfruit, dingto

Subscribers: venomgfx, hjalti

Differential Revision: https://developer.blender.org/D1380

intern/cycles/render/camera.cpp

@@ -29,6 +29,7 @@ CCL_NAMESPACE_BEGIN
 Camera::Camera()
 {
 	shuttertime = 1.0f;
+	motion_position = MOTION_POSITION_CENTER;
 
 	aperturesize = 0.0f;
 	focaldistance = 10.0f;