Exported maps are sorting/overlaying the images different from viewer

[gonzoearth]

largest images are winning on what image overlays are on top during export http://publiclaboratory.org/map/buss-farm-rosetta-south-africa/2011-10-08 as an example http://mapknitter.org/map/view/buss-farm

[jywarren]

yipes, there's actually no sorting on the export side. Plan:

• add an "images" field to the Export model, which accepts a comma-delimited list of Warpables (images)
• on starting an export, write to that field using IDs from the JavaScript side

This has the added benefit that we could offer a set of checkboxes to disable certain images, and eventually it could be the basis for manual sorting (which would take some extra javascript interface).

And it would help issue #39, by letting people try exporting subsets of the full map if it's too big.

Relevant code: app/models/map.rb:323 for sorting, pre-compositing