Blender Git Loki

Change Request: use weight centre of location tracks as pivot

Previously, this extension used the translation compensated image centre
as reference point for rotation measurement and compensation. During
user tests, it turned out that this setup tends to give poor results
with very simple track configurations.

This can be improved by useiing the weighted average of the location
tracks for each frame as pivot point. But there is a technical problem:
the existing public API functions do not allow to pass the pivot point
for each frame alongside with the stabilisation data. Thus this
change implements a trick to package a compensation shift into
the translation offset, so the rotation can be performed around
a fixed point (center of frame). The compensation shift will then shift
the image as if it had been rotated around the desired pivot point.