fixed a bug where a degenerate triangle causing the all polygons wrong orientation check to report false positives

CityDoctor Version 3.18.2

See merge request !40

Version 3.18.2

See merge request !39

Adds a generalized version of the 3D-Renderer LoadingDialog to improve
UX by showing that CityDoctor is currently busy doing something in the
background of indeterminate length, e.g. opening a file or waiting for
the Schematron thread to finish.