OpenRaster spec updated: 0.0.4-pre.1

[Andrew Chadwick]

I've updated the ORA spec to incorporate the changes in the revised
proposal[1], and bumped the version to the dizzying heights of
0.0.4-pre.1. Ever onward and upward, brave rocketeers!

http://www.freedesktop.org/wiki/Specifications/OpenRaster/Draft/
http://www.freedesktop.org/wiki/Specifications/OpenRaster/Draft/LayersStack/

It's not a vast set of changes really, and hopefully quite in keeping
with what apps out there already do:

* Layer groups can now be named.
* Layer groups can now take opacities and composite-ops, and act like
layers generally.
* For the first time we actually recommend a rendering model, which
will help deal with the can of worms this opens.
* Backwards-compatible, at least if you were doing *only* the simple
things the core spec allowed before.
* Two new composite-ops to make storing layer masks and group masks possible.

Is there anything more we need to do before this is finalized?
Personally I think getting some implementations out there and tested
would be of use, particularly with the mask stuff.

[1] https://gist.github.com/achadwick/7827931