Fix several problem with wxMemoryDC in wxQt

Under wxQT, wxMemoryDC was previously rendering to a temporary image
which was only being blitted back to the original wxBitmap when either
the DC wx destroyed or a new bitmap was selected (via SelectObject).

With these change wxMemoryDCImpl now draws directly to the bitmap
managed by wxBitmap, this makes the behaviour more consistent with the
MSW and GTK implementations.

Closes https://github.com/wxWidgets/wxWidgets/pull/1083

include/wx/qt/dc.h

@@ -111,10 +111,10 @@ public:
     virtual void* GetHandle() const { return (void*) m_qtPainter; }
 
 protected:
-    virtual QImage *GetQImage() { return m_qtImage; }
+    virtual QPixmap *GetQPixmap() { return m_qtPixmap; }
     
     QPainter *m_qtPainter;
-    QImage *m_qtImage;
+    QPixmap *m_qtPixmap;
 
     wxRegion *m_clippingRegion;
 private:

include/wx/qt/dcscreen.h

@@ -19,9 +19,7 @@ public:
 
 protected:
     virtual void DoGetSize(int *width, int *height) const wxOVERRIDE;
-    virtual bool DoGetPixel(wxCoord x, wxCoord y, wxColour *col) const;
-
-    virtual QImage *GetQImage();
+    virtual QPixmap *GetQPixmap() wxOVERRIDE;
 
     wxDECLARE_ABSTRACT_CLASS(wxScreenDCImpl);
 };