gizmos: draw shadow so they're still visible on white backdrop

Fixes #2142

app/node/gizmo/line.cpp

@@ -29,9 +29,16 @@ LineGizmo::LineGizmo(QObject *parent) :
 
 void LineGizmo::Draw(QPainter *p) const
 {
-  p->setBrush(Qt::NoBrush);
-  p->setPen(QPen(Qt::white, 0));
+  // Draw transposed black
+  QLineF transposed = p->transform().map(line_);
+  transposed.translate(1, 1);
+  transposed = p->transform().inverted().map(transposed);
+  p->setPen(QPen(Qt::black, 0));
+  p->drawLine(transposed);
 
+  // Draw normal white polygon
+  p->setPen(QPen(Qt::white, 0));
+  p->setBrush(Qt::NoBrush);
   p->drawLine(line_);
 }
 

app/node/gizmo/path.cpp

@@ -30,9 +30,16 @@ PathGizmo::PathGizmo(QObject *parent) :
 
 void PathGizmo::Draw(QPainter *p) const
 {
+  // Draw transposed black
+  QPainterPath transposed = p->transform().map(path_);
+  transposed.translate(1, 1);
+  transposed = p->transform().inverted().map(transposed);
+  p->setPen(QPen(Qt::black, 0));
+  p->drawPath(transposed);
+
+  // Draw normal white polygon
   p->setPen(QPen(Qt::white, 0));
   p->setBrush(Qt::NoBrush);
-
   p->drawPath(path_);
 }
 

app/node/gizmo/point.cpp

@@ -46,7 +46,7 @@ void PointGizmo::Draw(QPainter *p) const
   QRectF rect = GetDrawingRect(p->transform(), GetStandardRadius());
 
   if (shape_ != kAnchorPoint) {
-    p->setPen(Qt::NoPen);
+    p->setPen(QPen(Qt::black, 0));
     p->setBrush(Qt::white);
   }
 

app/node/gizmo/polygon.cpp

@@ -29,9 +29,16 @@ PolygonGizmo::PolygonGizmo(QObject *parent)
 
 void PolygonGizmo::Draw(QPainter *p) const
 {
+  // Draw transposed black
+  QPolygonF transposed = p->transform().map(polygon_);
+  transposed.translate(1, 1);
+  transposed = p->transform().inverted().map(transposed);
+  p->setPen(QPen(Qt::black, 0));
+  p->drawPolyline(transposed);
+
+  // Draw normal white polygon
   p->setPen(QPen(Qt::white, 0));
   p->setBrush(Qt::NoBrush);
-
   p->drawPolyline(polygon_);
 }