Fixes a bugs in the handling of the holdback mount point (by Mathias),

Fixes a bugs in the handling of the holdback mount point (by Mathias),
and in the calculation of the launchbar angle (by Vivian).

It also calculates the holdback angle, and sets a Boolean value which
can be used to initiate the release of the catapult strop submodel at
the appropriate moment (new code by Vivian).