-Add a function DCanvas::GetTransposedBlock to copy the screen buffer to another buffer, transposing it from row-major to column-major format.
-Use DCanvas::GetTransposedBlock when setting up the melt screen wipe.

git-svn-id: http://odamex.net/svn/root/trunk@3954 b0bcef46-2027-0410-bd56-ab726e9a1d04