QtCanvas: don't change canvas' visibility in init()

Previously, QtCanvas' init() would call show() on its underlying widget
and cause it to appear as a top-level window for a split second, before
the canvas was docked. To avoid this flicker, show() is now only called
on the canvas after it's been added to the MDI area.

ext/tgt/qt/qtcanvas.cpp

@@ -81,7 +81,6 @@ QtCanvas::QtCanvas(QWidget* parent, bool shared, Qt::WFlags f, char* /*name*/)
 QtCanvas::~QtCanvas() {}
 
 void QtCanvas::init() {
-    show();
     GLCanvas::init();
 }