only call XFreePixmap on pixmaps created via XCreatePixmap

not on those belonging to gdk shoe-horned in via

mpPixmap = GDK_PIXMAP_XID( mpGdkPixmap );

in GdkX11Pixmap::GdkX11Pixmap

This stops file->new->presentation crashing on second invocation

Change-Id: Iec673497f900b0ddabd439369b9ffe554800d425