Add "extra" xml attribute to fix bug of non-existent second texture

bandits have a second pirateflag texture that didn't exist in the config .xml
file. This is now available by specifying an "extra" xml attribute for a
secondary texture.

Also add getNumberFrameSeries() to double check that more than a single texture
exists. Otherwise a faulty config file can crash the renderer by trying to
access a non-existent texture.