If the XV image linesize was different from the width, SDL would create an
overlay wider than the actually requested one. This causes a greenish line on
the right, because the rightmost texel is blended with unset data.  This patch
introduces a function which duplicates the rightmost texel to the unset space,
that way the blending will be done with duplicated border texels.
Signed-off-by: Marton Balint <cus@passwd.hu>