Don't use wxWindow::ClearBackground() in the image sample and explain why.

Don't use wxWindow::ClearBackground() in the image sample and explain why.

Painting on both wxPaintDC and wxClientDC simultaneously doesn't work well,
e.g. under Windows the client DC can be actually erased after we finished
painting the window contents, overwriting it.

Simply use wxDC::Clear() instead of wxWindow::ClearBackground() to avoid this
and document the danger of using ClearBackground() from EVT_PAINT handler.

Closes #10700.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64277 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775