mnUsedSize was never decreased causing the cache to pretending to be full,
which caused lots of needless work (image loading, id creation (which are
basically a crc of all bytes of an image, ...))

Change-Id: I1e0a6a07631435346c3509822fae43a067d33303
Reviewed-on: https://gerrit.libreoffice.org/14389Reviewed-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Zolnai Tamás <zolnaitamas2000@gmail.com>
Tested-by: Zolnai Tamás <zolnaitamas2000@gmail.com>