Make QtGetScrollBarsContainer public for wxClientDC
Repaint only the rect affected and handle scroll area
Don't initialize the Qt Picture size

This avoids setting an excesive update region that was clearing the whole widget in the paint event handler.
Now, wxClientDC will only paint the effective drawn rect.
Divide wxClientDC & wxPaintDC logic in paint event handler

wxClientDC should not erase the background nor generate a wxPaintEvent, it only replay the QPicture painted outside the event handler.
This way was originally designed but it was not working in prior versions due lacking update region & background support.
Prevent Qt to auto-erase the background on paint events

This is need by wxClientDC to be able to draw outside the paint event.
Now this is set by default now for all the widgets but can be changed by SetBackgroundStyle (so it will need to be revised that background erasing is working properly).
Add basic logging facilities to debug wxQT paint issues