general: fix compiler warning about object construction and assignment

TextWindow uses memset to zero objects that have non-trivial constructors,
in order to avoid assignment with a temporary which leads to an overflow
of the stack. Replace it with an explicit destructor call and placement
new.

Author: iscgar

src/textwin.cpp

@@ -3,6 +3,7 @@
 //
 // Copyright 2008-2013 Jonathan Westhues.
 //-----------------------------------------------------------------------------
+#include <new>
 #include "solvespace.h"
 
 namespace SolveSpace {
@@ -283,16 +284,17 @@ void TextWindow::Init() {
 void TextWindow::ClearSuper() {
     // Ugly hack, but not so ugly as the next line
     Platform::WindowRef oldWindow = std::move(window);
-    std::shared_ptr<ViewportCanvas> oldCanvas = canvas;
+    std::shared_ptr<ViewportCanvas> oldCanvas = std::move(canvas);
 
     // Cannot use *this = {} here because TextWindow instances
     // are 2.4MB long; this causes stack overflows in prologue
     // when built with MSVC, even with optimizations.
-    memset(this, 0, sizeof(*this));
+    this->~TextWindow();
+    new(this) TextWindow();
 
     // Return old canvas
     window = std::move(oldWindow);
-    canvas = oldCanvas;
+    canvas = std::move(oldCanvas);
 
     HideEditControl();