Manifests without 'scale_factors' field

[rsimon]

I have encountered a number of info.json files that don't specify a scale_factors array. See e.g.

http://diglit.ub.uni-heidelberg.de/image/salXIV5_1/a.jpg/info.json

IIIFViewer seems to expect this to be present, either at the top level or in a tiles object. The part where it breaks for me as at the initialization stage of the source, where the `logicalResolutions' array is reversed (but is undefined).

The images seem to display fine in other viewers. Any hints on how this might be fixed?