Fix print preview rendered page preloading.

After the preview list is scrolled we are preloading the
pages around the ones that are shown, so now if the user
does a slight adjustment (expected after a scroll) the
pages around are rendered. There was an off by one error
and one of the bitmaps of the shown pages was reused and
as a result the wrong content was shown.

Change-Id: If04aebf46bcc637491221c6218005ecc1a7253d2