fdo#81592 Use 24-bit color depth, not 256 colors when converting an EPS-file.

This regression was introduced in f5c3f5601a3739dead635f9abc446951b385018f.

Change-Id: Ica48732398cd5d225b433b177ff6e6fff649e732
(cherry picked from commit 0791335cf007e04de25e03c0a6e0aa53fa66016a)
Reviewed-on: https://gerrit.libreoffice.org/11464
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>