Notes on OpenRaster support in Drawpile

As of version 0.7, Drawpile defaults to OpenRaster format for images containing
annotations or more than one layer. (The default for simple images being PNG)

The standard is still at a draft stage and subject to change, and so is
Drawpile's implementation of it.

Presently, a Drawpile specific extension is used for saving the annotations.
They are saved under the <drawpile:annotations> element.
Once the <text> element of OpenRaster is standardised, it will probably be used instead.

A drawpile specific layer attribute `drawpile:censored` is set to `true` if the layer
has been flagged as censored.

Additionally, Drawpile partially supports MyPaint's background layer extension.
If the bottom-most layer has an attribute named `background-tile` (in namespace `http://mypaint.org/ns/openraster`)
and the background tile is 64x64 pixels in size, that layer is ignored and the background tile
image is set as the canvas background. When saving an ORA image and the canvas background is non-transparent,
a filled background layer is generated with the `background-tile` attribute set.

The animation timeline (if used) is saved under a Drawpile specific element `<drawpile:timeline>`.
It contains `<frame>` elements, which again contain a list of layer indexes, where 0 is the topmost layer.

Compatability has been tested with the following programs:
 * MyPaint
 * GIMP
 * Krita

Further reading
 * http://www.freedesktop.org/wiki/Specifications/OpenRaster/

