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

This regression was introduced in f5c3f5601a3739dead635f9abc446951b385018f.

Change-Id: Ica48732398cd5d225b433b177ff6e6fff649e732
Reviewed-on: https://gerrit.libreoffice.org/11268
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>