cause the disabled color cannot be mapped to the 2 entries in the
palette, so use 8bit dests if the src is 1bit

regression from

commit 507d0ded
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Tue Dec 1 12:18:57 2015 +0000

    keep disable image at same depth as original

seen when clicking on the image loaded from tdf#100632

Change-Id: Ic69bccfa0fc86707b4fd9e757d8374d80e4b0071