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