Allow disabling snap-to when placing vias

This allows the user to selectively disable the snap-to behavior when
placing stand-alone vias.  Full solution will require an update to the
segment distance calculation that takes into account the rounded line
caps.

Fixes: lp:1769523
* https://bugs.launchpad.net/kicad/+bug/1769523

pcbnew/tools/drawing_tool.cpp

@@ -1518,15 +1518,20 @@ int DRAWING_TOOL::DrawVia( const TOOL_EVENT& aEvent )
 
         void SnapItem( BOARD_ITEM *aItem ) override
         {
-#if 0
             // If you place a Via on a track but not on its centerline, the current
             // connectivity algorithm will require us to put a kink in the track when
             // we break it (so that each of the two segments ends on the via center).
             // That's not ideal, and is in fact probably worse than forcing snap in
             // this situation.
-            if( m_frame->Settings().m_magneticTracks == CAPTURE_CURSOR_IN_TRACK_TOOL
-                || m_frame->Settings().m_magneticTracks == CAPTURE_ALWAYS )
-#endif
+
+//          bool do_snap = ( m_frame->Settings().m_magneticTracks == CAPTURE_CURSOR_IN_TRACK_TOOL
+//                || m_frame->Settings().m_magneticTracks == CAPTURE_ALWAYS );
+            bool do_snap = true;
+
+            if( m_modifiers & MD_SHIFT )
+                do_snap = !do_snap;
+
+            if( do_snap )
             {
                 auto    via = static_cast<VIA*>( aItem );
                 wxPoint pos = via->GetPosition();

pcbnew/tools/pcb_tool.cpp

@@ -59,6 +59,7 @@ void PCB_TOOL::doInteractiveItemPlacement( INTERACTIVE_PLACER_BASE* aPlacer,
 
     aPlacer->m_board = board();
     aPlacer->m_frame = frame();
+    aPlacer->m_modifiers = 0;
 
     if( aOptions & IPO_SINGLE_CLICK )
     {
@@ -74,6 +75,7 @@ void PCB_TOOL::doInteractiveItemPlacement( INTERACTIVE_PLACER_BASE* aPlacer,
     while( OPT_TOOL_EVENT evt = Wait() )
     {
         VECTOR2I cursorPos = controls()->GetCursorPosition();
+        aPlacer->m_modifiers = evt->Modifier();
 
         if( TOOL_EVT_UTILS::IsCancelInteractive( *evt ) )
         {

pcbnew/tools/pcb_tool.h

@@ -57,6 +57,7 @@ struct INTERACTIVE_PLACER_BASE
 
     PCB_EDIT_FRAME* m_frame;
     BOARD* m_board;
+    int m_modifiers;
 };