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.h

@@ -32,12 +32,24 @@ class Scene;
 /* Camera
  *
  * The camera parameters are quite standard, tested to be both compatible with
- * Renderman, and Blender after remapping. */
+ * Renderman, and Blender after remapping.
+ */
 
 class Camera {
 public:
+	/* Specifies an offset for the shutter's time interval. */
+	enum MotionPosition {
+		/* Shutter opens at the current frame. */
+		MOTION_POSITION_START,
+		/* Shutter is fully open at the current frame. */
+		MOTION_POSITION_CENTER,
+		/* Shutter closes at the current frame. */
+		MOTION_POSITION_END,
+	};
+
 	/* motion blur */
 	float shuttertime;
+	MotionPosition motion_position;
 
 	/* depth of field */
 	float focaldistance;