Keep track of the area intended to be buffered, and use that in UnMask instead of...

Keep track of the area intended to be buffered, and use that in UnMask instead of defaulting to the buffer size.  Only clip to the DC size if wxBUFFER_VIRTUAL_AREA is not set.  This fixes the issue where buffering the full virtual area in a wxScrolledWindow would only draw an area the physical size of the window.  Fixes #11612.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71704 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775