gtk_clist_set_pixmap and gtk_clist_set_pixtext requires to use a mask,
when this is not necesary in most cases. This patch is to allow the mask
argument in these functions to be NULL. If this is the case, it will not
_ref/unref the bitmap mask and will not set the clip_mask to the gc
before drawing the pixmap.

        If you want to test it, you can change in testgtk the line:

   gtk_clist_set_pixtext (GTK_CLIST (data), row, 3, "Testing", 5,
pixmap, mask); 

        to:

   gtk_clist_set_pixtext (GTK_CLIST (data), row, 3, "Testing", 5,
pixmap, NULL); 

        testgtk needs the mask to look right, I sugest the change only to test
this patch, not to be permanent.