Change Zone extra clearance magic number to AC

Rather than keeping a magic number in the code, we set the default in
the advanced config setting and allow users/developers to modify it.

common/advanced_config.cpp

@@ -60,12 +60,12 @@ namespace AC_STACK
  */
 namespace AC_KEYS
 {
+
 /**
- * In Pcbnew, pads can have a fabrication property
- * Because this feature adds a new keyword in *.kicad_pcb and *.kicad_modfiles,
- * this is an advanced feature until it is fully finalized
+ * When filling zones, we add an extra amount of clearance to each zone to ensure that rounding
+ * errors do not overrun minimum clearance distances.  This is the extra in mm.
  */
-static const wxChar UsePadProperty[] = wxT( "UsePadProperty" );
+static const wxChar ExtraFillMargin[] = wxT( "ExtraFillMargin" );
 
 /**
  * Testing mode for new connectivity algorithm.  Setting this to on will cause all modifications
@@ -201,6 +201,9 @@ void ADVANCED_CFG::loadSettings( wxConfigBase& aCfg )
     configParams.push_back( new PARAM_CFG_BOOL( true, AC_KEYS::RealtimeConnectivity,
                                                 &m_realTimeConnectivity, false ) );
 
+    configParams.push_back( new PARAM_CFG_DOUBLE( true, AC_KEYS::ExtraFillMargin,
+                                                  &m_extraClearance, 0.002, 0.0, 1.0 ) );
+
     configParams.push_back( new PARAM_CFG_INT( true, AC_KEYS::CoroutineStackSize,
                                                &m_coroutineStackSize, AC_STACK::default_stack,
                                                AC_STACK::min_stack, AC_STACK::max_stack ) );

include/advanced_config.h

@@ -68,6 +68,11 @@ public:
      */
     static const ADVANCED_CFG& GetCfg();
 
+    /**
+     * Extra fill clearance for zone fills
+     */
+    double m_extraClearance;
+
     /**
      * Do real-time connectivity
      */

pcbnew/zone_filler.cpp

@@ -27,6 +27,7 @@
 #include <algorithm>
 #include <future>
 
+#include <advanced_config.h>
 #include <class_board.h>
 #include <class_zone.h>
 #include <class_module.h>
@@ -560,7 +561,7 @@ void ZONE_FILLER::buildCopperItemClearances( const ZONE_CONTAINER* aZone, PCB_LA
     // than requested clearance due to many approximations in calculations,
     // like arc to segment approx, rounding issues...
     // 2 microns are a good value
-    int extra_margin = Millimeter2iu( 0.002 );
+    int extra_margin = Millimeter2iu( ADVANCED_CFG::GetCfg().m_extraClearance );
 
     BOARD_DESIGN_SETTINGS& bds = m_board->GetDesignSettings();
     int                    zone_clearance = aZone->GetLocalClearance();