support mat_ids in merge_mesh()
support mat_ids in merge_mesh()

RexFuzzle/sfepy,rc/sfepy,sfepy/sfepy,BubuLK/sfepy,rc/sfepy,vlukes/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,lokik/sfepy,sfepy/sfepy,lokik/sfepy,lokik/sfepy,sfepy/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,lokik/sfepy,rc/sfepy,vlukes/sfepy