Create processing task that exports or pre-renders an image.
Returns may contain field 'note' with value
'ALREADY_EXISTS' if an identical task with the same ID already exists. Null if a callback is specified.
ID for the task (obtained using newTaskId).
The object that describes the processing task; only fields that are common for all processing types are documented here. type (string) Either 'EXPORT_IMAGE', 'EXPORT_FEATURES', 'EXPORT_VIDEO' or 'EXPORT_TILES'. json (string) JSON description of the image.
An optional callback. If not supplied, the call is made synchronously.