The bitmap must be deselected from wxMemoryDC before being used in any other
way but the sample didn't do this. Fix this by simply destroying the DC as
soon as we don't need it, this makes bitmap available for other use as well.
Closes#12310.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65232 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775