MyPaint needs to store a number of additional fields in OpenRaster files
for its own benefit. The natural way of doing this in XML is to use a
namespace, so I've picked "http://mypaint.org/ns/openraster" - which isn't
an addressable resource yet, but it a) doesn't have to be, and b) could be
one day if it's useful to have something there.
Okay, OpenRaster doesn't even have its own xmlns yet, but no reason not to
be ahead of the curve.
Save a custom xsd:bool declaring whether the frame is on or off in an XML
attribute in OpenRaster files and autosave oradirs. It's a distinct
concept from image size, and shouldn't be confused with it.
Update the frame-enabled state from the saved value when loading OpenRaster
files or oradirs files. The default is no frame (infinite canvas).