Blender Git Loki

Fix T66610: Planar Track extremely laggy when 3D View is open

The issue was caused by modifications to planar track tagging clip for
copy-on-write, which was invalidating its cache and forcing current
frame in 3D viewport to be re-load.

Ideal solution would be to share movie cache across original and
evaluated movie clips which will reduce memory usage. However, doing
such ownership changes so close to the code freeze is not something
comfortable to do.