Allow DXF to be exported in either inches or millimeters

Fixes: lp:1832188
* https://bugs.launchpad.net/kicad/+bug/1832188

pcbnew/pcb_plot_params.cpp

@@ -103,6 +103,7 @@ PCB_PLOT_PARAMS::PCB_PLOT_PARAMS()
     m_plotViaOnMaskLayer         = false;
     m_plotMode                   = FILLED;
     m_DXFplotPolygonMode         = true;
+    m_DXFplotUnits               = INCHES;
     m_useAuxOrigin               = false;
     m_HPGLPenNum                 = 1;
     m_HPGLPenSpeed               = 20;        // this param is always in cm/s
@@ -260,7 +261,9 @@ bool PCB_PLOT_PARAMS::IsSameAs( const PCB_PLOT_PARAMS &aPcbPlotParams, bool aCom
     if( !aCompareOnlySavedPrms )
     {
         if( m_DXFplotPolygonMode != aPcbPlotParams.m_DXFplotPolygonMode )
-        return false;
+            return false;
+        if( m_DXFplotUnits != aPcbPlotParams.m_DXFplotUnits )
+            return false;
     }
     if( m_useAuxOrigin != aPcbPlotParams.m_useAuxOrigin )
         return false;