To prepare for an alignment fix that requires to know the ratio of the
image, let's add a ratio property.
This property is calling `gm identify` which is already a dependency of
prosopopee.
The output of this particular `gm identify` returns the width and height
separated by a comma.
Signed-off-by: Quentin Schulz <foss@0leil.net>
8b4f645a65