Fix T93995: Cycles camera motion blur not working in right stereo view

Thanks to Michael (michael64) for identifying the solution.

Ref D13567

intern/cycles/blender/session.cpp

@@ -396,6 +396,13 @@ void BlenderSession::render(BL::Depsgraph &b_depsgraph_)
     /* set the current view */
     b_engine.active_view_set(b_rview_name.c_str());
 
+    /* Force update in this case, since the camera transform on each frame changes
+     * in different views. This could be optimized by somehow storing the animated
+     * camera transforms separate from the fixed stereo transform. */
+    if ((scene->need_motion() != Scene::MOTION_NONE) && view_index > 0) {
+      sync->tag_update();
+    }
+
     /* update scene */
     BL::Object b_camera_override(b_engine.camera_override());
     sync->sync_camera(b_render, b_camera_override, width, height, b_rview_name.c_str());

intern/cycles/blender/sync.cpp

@@ -95,6 +95,11 @@ void BlenderSync::reset(BL::BlendData &b_data, BL::Scene &b_scene)
   this->b_scene = b_scene;
 }
 
+void BlenderSync::tag_update()
+{
+  has_updates_ = true;
+}
+
 /* Sync */
 
 void BlenderSync::sync_recalc(BL::Depsgraph &b_depsgraph, BL::SpaceView3D &b_v3d)

intern/cycles/blender/sync.h

@@ -66,6 +66,8 @@ class BlenderSync {
 
   void reset(BL::BlendData &b_data, BL::Scene &b_scene);
 
+  void tag_update();
+
   /* sync */
   void sync_recalc(BL::Depsgraph &b_depsgraph, BL::SpaceView3D &b_v3d);
   void sync_data(BL::RenderSettings &b_render,
@@ -267,7 +269,6 @@ class BlenderSync {
 
   Progress &progress;
 
- protected:
   /* Indicates that `sync_recalc()` detected changes in the scene.
    * If this flag is false then the data is considered to be up-to-date and will not be
    * synchronized at all. */