Gui: fix some MSVC truncation warnings

See forum: https://forum.freecad.org/viewtopic.php?t=81051

src/Gui/MainWindow.cpp

@@ -1734,7 +1734,7 @@ void MainWindow::renderDevBuildWarning(
         lineHeight = painter.fontMetrics().lineSpacing();
         boxWidth = maxSize.width();
     }
-    constexpr float lineExpansionFactor(2.3);
+    constexpr float lineExpansionFactor(2.3F);
     int boxHeight = static_cast<int>(lineHeight*lineExpansionFactor);
 
     // Draw the background rectangle and the text

src/Gui/Selection.h

@@ -531,10 +531,16 @@ public:
      */
     //@{
     /// Return the current selection stack size
-    int selStackBackSize() const {return _SelStackBack.size();}
+    std::size_t selStackBackSize() const
+    {
+        return _SelStackBack.size();
+    }
 
     /// Return the current forward selection stack size
-    int selStackForwardSize() const {return _SelStackForward.size();}
+    std::size_t selStackForwardSize() const
+    {
+        return _SelStackForward.size();
+    }
 
     /** Obtain selected objects from stack
      *

src/Gui/View3DInventorViewer.cpp

@@ -1333,7 +1333,7 @@ void View3DInventorViewer::showRotationCenter(bool show)
 
             auto material = new SoMaterial();
             material->emissiveColor = SbColor(1, 0, 0);
-            material->transparency = 0.8;
+            material->transparency = 0.8F;
 
             auto translation = new SoTranslation();
             translation->translation.setValue(center);