fix Viewer.save_image() for file names with path
fix Viewer.save_image() for file names with path

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