BOARD: add helper function GetCopperLayerStackMaxId() and remove dead code

GetCopperLayerStackMaxId() replaces GetCopperLayerCount()*2 and is more
understandable in code where the max allowed PCB_LAYER_ID value is needed.
Removed also a dead code related to GetCopperLayerCount() < 2, broken and not
allowed in Kicad.

pcbnew/board.cpp

@@ -745,6 +745,19 @@ void BOARD::SetCopperLayerCount( int aCount )
 }
 
 
+PCB_LAYER_ID BOARD::GetCopperLayerStackMaxId() const
+{
+    int imax = GetCopperLayerCount();
+
+    // layers IDs are F_Cu, B_Cu, and even IDs values (imax values)
+    if( imax <= 2 )     // at least 2 layers are expected
+        return B_Cu;
+
+    // For a 4 layer, last ID is In2_Cu = 6 (IDs are 0, 2, 4, 6)
+    return static_cast<PCB_LAYER_ID>( (imax-1) * 2 );
+}
+
+
 int BOARD::LayerDepth( PCB_LAYER_ID aStartLayer, PCB_LAYER_ID aEndLayer ) const
 {
     if( aStartLayer > aEndLayer )

pcbnew/board.h

@@ -570,6 +570,12 @@ public:
     int  GetCopperLayerCount() const;
     void SetCopperLayerCount( int aCount );
 
+    /**
+     * @return The copper layer max PCB_LAYER_ID in the BOARD.
+     * similar to GetCopperLayerCount(), but returns the max PCB_LAYER_ID
+     */
+    PCB_LAYER_ID  GetCopperLayerStackMaxId() const;
+
     PCB_LAYER_ID FlipLayer( PCB_LAYER_ID aLayer ) const;
 
     int LayerDepth( PCB_LAYER_ID aStartLayer, PCB_LAYER_ID aEndLayer ) const;

pcbnew/edit.cpp

@@ -94,21 +94,8 @@ void PCB_EDIT_FRAME::SwitchLayer( PCB_LAYER_ID layer )
     // enabled needs to be checked.
     if( IsCopperLayer( layer ) )
     {
-        // If only one copper layer is enabled, the only such layer that can be selected to is
-        // the "Back" layer (so the selection of any other copper layer is disregarded).
-        if( GetBoard()->GetCopperLayerCount() < 2 )
-        {
-            if( layer != B_Cu )
-                return;
-        }
-        // If more than one copper layer is enabled, the "Copper" and "Component" layers can be
-        // selected, but the total number of copper layers determines which internal layers are
-        // also capable of being selected.
-        else
-        {
-            if( layer != B_Cu && layer != F_Cu && layer >= GetBoard()->GetCopperLayerCount()*2 )
-                return;
-        }
+        if( layer > GetBoard()->GetCopperLayerStackMaxId() )
+            return;
     }
 
     // Is yet more checking required? E.g. when the layer to be selected is a non-copper layer,

pcbnew/pcb_base_frame.cpp

@@ -723,22 +723,8 @@ void PCB_BASE_FRAME::SwitchLayer( PCB_LAYER_ID layer )
     // currently enabled needs to be checked.
     if( IsCopperLayer( layer ) )
     {
-        // If only one copper layer is enabled, the only such layer that can be selected to
-        // is the "Copper" layer (so the selection of any other copper layer is disregarded).
-        if( m_pcb->GetCopperLayerCount() < 2 )
-        {
-            if( layer != B_Cu )
-                return;
-        }
-
-        // If more than one copper layer is enabled, the "Copper" and "Component" layers
-        // can be selected, but the total number of copper layers determines which internal
-        // layers are also capable of being selected.
-        else
-        {
-            if( layer != B_Cu && layer != F_Cu && layer >= m_pcb->GetCopperLayerCount()*2 )
-                return;
-        }
+        if( layer > m_pcb->GetCopperLayerStackMaxId() )
+            return;
     }
 
     // Is yet more checking required? E.g. when the layer to be selected is a non-copper