Gerber plot: new algo to generate solder mask layers with non 0 min thickness mask

Previously, all overlapping polygons (pads and min thickness areas to remove) were
merged.
Drawback: pads attributes are lost. In Gerber this is annoying.
Now the pads are plot as flashed or Regions items, and min thickness areas are added
but shapes are not merged and keep their attributes.

pcbnew/plot_brditems_plotter.cpp

@@ -75,7 +75,7 @@ COLOR4D BRDITEMS_PLOTTER::getColor( LAYER_NUM aLayer )
 {
     COLOR4D color = m_colors->GetColor( aLayer );
 
-    // A hack to avoid plotting ahite itmen in white color, expecting the paper
+    // A hack to avoid plotting a white item in white color, expecting the paper
     // is also white: use a non white color:
     if( color == COLOR4D::WHITE )
         color = COLOR4D( LIGHTGRAY );