Fix painting logic in wxQt wxClientDC implementation

The original drawing mechanism was generating lots of QWarning messages
when running samples (e.g. htlbox, caret, etc.) and in some cases was
not actually completely drawing every element of the sample. The issue
was that the QPicture was being shared incorrectly between wxWindow and
wxClientDC and attempts to start painting, update, etc. were generating
console warnings.

Closes https://github.com/wxWidgets/wxWidgets/pull/1152

src/qt/window.cpp

@@ -210,7 +210,7 @@ void wxWindowQt::Init()
     m_horzScrollBar = NULL;
     m_vertScrollBar = NULL;
 
-    m_qtPicture = new QPicture();
+    m_qtPicture = NULL;
     m_qtPainter = new QPainter();
 
     m_mouseInside = false;
@@ -251,7 +251,6 @@ wxWindowQt::~wxWindowQt()
 
     DestroyChildren(); // This also destroys scrollbars
 
-    delete m_qtPicture;
     delete m_qtPainter;
 
 #if wxUSE_ACCEL
@@ -1126,7 +1125,7 @@ bool wxWindowQt::QtHandlePaintEvent ( QWidget *handler, QPaintEvent *event )
         {
             bool handled;
 
-            if ( m_qtPicture->isNull() )
+            if ( m_qtPicture == NULL )
             {
                 // Real paint event (not for wxClientDC), prepare the background
                 switch ( GetBackgroundStyle() )
@@ -1558,9 +1557,9 @@ QScrollArea *wxWindowQt::QtGetScrollBarsContainer() const
     return m_qtContainer;
 }
 
-QPicture *wxWindowQt::QtGetPicture() const
+void wxWindowQt::QtSetPicture( QPicture* pict )
 {
-    return m_qtPicture;
+    m_qtPicture = pict;
 }
 
 QPainter *wxWindowQt::QtGetPainter()