When you hit "setup tracking scene" in MCE, the active layer usually switches
to the layer where the ground has been created, thus he shadow layer.
New objects are created or appended on that layer which means they will be
default be rendered as shadows only. This behaviour requires muscle memory
and is confusing for new users. This patch is changing the behaviour in a
way that the active layer is remembered from before setting up the scene.
Patch by gottfried, thanks!
Reviewers: sebastian_k, sergey
Differential Revision: https://developer.blender.org/D574
559e3db792