Blender Git Loki

Support normalization in the tracking prepass

The last tracker commit added normalized tracking. This makes
tracking patches undergoing uniform illumination change easier.
However, the prepass which computes a quick translation-only
estimate of the warp did not take this into account. This commit
fixes that.

This works reasonably well but in some examples the brute
initialization fails. I suspect this is due to the warped template
estimate in the current frame being too different from the
original, so there are multiple peaks in the normalized-SAD
correlation function.

The solution is to use the previous frame for the brute
initialization and the keyframe for refinement, but that requires
architecture changes.