ResizeRatio: allow client to provide it directly

This was driven because node-markup was always returning 'crop'
information in the markup string.

Include the ratio directly so we can also send a null crop instruction
if it doesn't exist. Because the alternative doesn't make much sense:

  *also* include the real image source width and height (when crop
  isn't present) but only use the width and only use it when crop isn't
  present (seems strange).