Fix GEOS errors from self intersecting polygons in and fix OPENCV path in debian CI build

This MR fixes the bug occurring on debian unstable

GEOS 3.8 throws an error when using the intersection method with self intersecting polygons (this is not the case in e.g. GEOS 3.6). OGRLayerStreamStitchingFilter has been modified to test polygon validity before fusion. non-valid polygon are not candidate for fusion anymore. The baselines of the test of this filter (and the segmentation application) have been updated.

Debian unstable now uses opencv 4.1.2

The layout of the include dir is different compared to opencv 3. The FindCmake is supposed to handle both cases. However in the CI debian script the opencv include dir is hardcoded.