Fix T41532: Some files bounce back and forth between 'packing BVH nodes' and 'Copying Transforms to Device'

This was originally caused by a6ae12a where i didn't foresee unclear distinguishing
between empty and non-synced meshes will give issues for the viewport. They're the
same for final rendering, but for viewport we need to be accurate here.

intern/cycles/render/mesh.cpp

@@ -132,6 +132,7 @@ void Mesh::clear()
 	transform_applied = false;
 	transform_negative_scaled = false;
 	transform_normal = transform_identity();
+	geometry_synced = false;
 }
 
 int Mesh::split_vertex(int vertex)