fdo#77140 embeddedobj: avoid unneeded conversion in ConvertBufferToFormat()

The primary reason is not performance: on export currently we loose some
EMF+ properties. While it would be good to fix all those problems one by
one, this is a small fix to avoid all kind of roundtrip problems in this
situation.

Change-Id: If5e1c1eabd8290f36b538e374c9707ae17097786
(cherry picked from commit 2a7fdf2a7bb7345c49c988dfd1fcdb41cebb8a1e)
fdo#77229 EMF+ rendering: improve EmfPlusSetClipPath's CombineModeExclude case

This is still not perfect, but at least we now don't do the opposite of
what was asked.

Change-Id: I5e144c5ec2987902e65b2eb472259d9c39bbbd11
(cherry picked from commit c2af50eb6df396c957890a6b912b8f3185893551)