Fix solder mask plotting issues.

1) Don't plot all footprint texts for each footprint graphic item.

2) Enforce min web thickness around board shapes and board text.

3) Enforce min web thickness around footprint shapes

4) Correctly handle multi-layer zones.

(1) and (4) will get cherry-picked back to 6.0, so after rebasing
won't actually appear in this changelist anymore....

pcbnew/plot_brditems_plotter.cpp

@@ -340,36 +340,40 @@ void BRDITEMS_PLOTTER::PlotFootprintTextItems( const FOOTPRINT* aFootprint )
 }
 
 
+void BRDITEMS_PLOTTER::PlotPcbGraphicItem( const BOARD_ITEM* item )
+{
+    switch( item->Type() )
+    {
+    case PCB_SHAPE_T:
+        PlotPcbShape( static_cast<const PCB_SHAPE*>( item ) );
+        break;
+
+    case PCB_TEXT_T:
+        PlotPcbText( static_cast<const PCB_TEXT*>( item ) );
+        break;
+
+    case PCB_DIM_ALIGNED_T:
+    case PCB_DIM_CENTER_T:
+    case PCB_DIM_RADIAL_T:
+    case PCB_DIM_ORTHOGONAL_T:
+    case PCB_DIM_LEADER_T:
+        PlotDimension( static_cast<const PCB_DIMENSION_BASE*>( item ) );
+        break;
+
+    case PCB_TARGET_T:
+        PlotPcbTarget( static_cast<const PCB_TARGET*>( item ) );
+        break;
+
+    default:
+        break;
+    }
+}
+
+
 void BRDITEMS_PLOTTER::PlotBoardGraphicItems()
 {
-    for( BOARD_ITEM* item : m_board->Drawings() )
-    {
-        switch( item->Type() )
-        {
-        case PCB_SHAPE_T:
-            PlotPcbShape( (PCB_SHAPE*) item );
-            break;
-
-        case PCB_TEXT_T:
-            PlotPcbText( (PCB_TEXT*) item );
-            break;
-
-        case PCB_DIM_ALIGNED_T:
-        case PCB_DIM_CENTER_T:
-        case PCB_DIM_RADIAL_T:
-        case PCB_DIM_ORTHOGONAL_T:
-        case PCB_DIM_LEADER_T:
-            PlotDimension( (PCB_DIMENSION_BASE*) item );
-            break;
-
-        case PCB_TARGET_T:
-            PlotPcbTarget( (PCB_TARGET*) item );
-            break;
-
-        default:
-            break;
-        }
-    }
+    for( const BOARD_ITEM* item : m_board->Drawings() )
+        PlotPcbGraphicItem( item );
 }