Workaround: disable track weights for initialisation

this workaround allows to use track weights in practice, while the proper
access to keyframed values is not yet implemented.

Explanation: for initialisation, we need to pick the measured values at
various reference frames, which are obviously not at the global "current frame"
position and thus don't hold the correct keyframed values. Typically this leads
to baseline values not being picked up at all (when the respective tracks
have been faded out through animation)