Pcbnew, zone filler: Ensure hatch pattern thickness bigger than zone minimal width (one micron bigger).

This margin is mandatory to avoid incorrect polygons in Gerber files:
due to the fact Gerber units can be 10 nm (and Gerbview internal unit is also 10nm), valid polygons
can be converted to non valid (self intersecting for instance) by coordinates truncation.
This is a known problem in Gerber files (not specific to Gerber format)

Fixes: lp:1847737
https://bugs.launchpad.net/kicad/+bug/1847737

pcbnew/zone_filler.cpp

@@ -48,6 +48,7 @@
 #include <geometry/convex_hull.h>
 #include <geometry/geometry_utils.h>
 #include <confirm.h>
+#include <convert_to_biu.h>
 
 #include "zone_filler.h"
 
@@ -995,10 +996,15 @@ void ZONE_FILLER::addHatchFillTypeOnZone( const ZONE_CONTAINER* aZone, SHAPE_POL
 {
     // Build grid:
 
-    // obvously line thickness must be > zone min thickness. However, it should be
-    // the case because the zone dialog setup ensure that. However, it can happens
-    // if a board file was edited by hand by a python script
-    int thickness = std::max( aZone->GetHatchFillTypeThickness(), aZone->GetMinThickness()+2 );
+    // obviously line thickness must be > zone min thickness.
+    // It can happens if a board file was edited by hand by a python script
+    // Use 1 micron margin to be *sure* there is no issue in Gerber files
+    // (Gbr file unit = 1 or 10 nm) due to some truncation in coordinates or calculations
+    // This margin also avoid problems due to rounding coordinates in next calculations
+    // that can create incorrect polygons
+    int thickness = std::max( aZone->GetHatchFillTypeThickness(),
+                              aZone->GetMinThickness()+Millimeter2iu( 0.001 ) );
+
     int linethickness = thickness - aZone->GetMinThickness();
     int gridsize = thickness + aZone->GetHatchFillTypeGap();
     double orientation = aZone->GetHatchFillTypeOrientation();