fdo#74702 Allow Window & OutputDevice to handle erasure

Window handles erasure of backgrounds for controls. Probably this
needs to be moved to a more specific class in the future, but for
now it should go into Windows.

I also removed an unnecessary temporary variable, and removed the
need to case the pointer to this from OutputDevice to Window. In
other words, this code has been vastly simplified :-)

Change-Id: I44b91cf68f29424ffbc1679b66fbeaeb024880e9