patch for gr_UnixImage.cpp

Hi,

recent changes to gr_UnixImage.cpp (1.39->1.40) broke charts. Here is a
patch which fixes that.
I found another problem: changing the height of the chart (or another
embedded object) does not change ascent and descent, and descent is
calculated with :
     iDescent = UT_convertToLogicalUnits(pszHeight) - iAscent;
so it can become negative... IMHO, both ascent and descent should be
changed proportionally when height change.
This can be done easily in fp_EmbedRun.cpp. Any comment?

Cheers,
Jean