The distance between curve point and control point was calculated
as linear scaling with ratio of central angle of the arc to pi/2.
The patch replaces this with the mathematical correct formula.

Change-Id: I0195e40bb99cd4bbcce95f7ac896158b7e7e3d29
Reviewed-on: https://gerrit.libreoffice.org/63659
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>