Paint directly wxWindowDC without buffer in wxQT

This solution avoid flicker and is more optimal as the intermediate buffer is not need:
 * use QPainter* instead of QImage* in wxWindow paint event (no more m_qtPaintBuffer)
 * change m_qtPainter to pointer in wxQtDCImpl (created on subclasses)
 * rename QtPreparePainter internal method (not used right now)
 * initial erase left to Qt (auto fill background)

Still more tweaks are needed to avoid problems on begin() and test background erasing