Erase the entire virtual area of the window in the erase sample.

Erase the entire virtual area of the window in the erase sample.

Just clearing the DC is not enough when the window is scrolled, so clear the
entire virtual area. We should be able to optimize it by clearing just the
rectangle currently scrolled into view but this is at least correct, i.e.
doesn't result in corrupted display, even if it's suboptimal.

See #14917.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73493 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775