This was a consequence of implementing the "-unique" flag. It
required adding a "data-dir" flag to the command line which
through off the _very_ brittle arg parser in the "open" event
callback.

This makes it possible to define an editable viewport outside of which the
contents are clipped.

One big benefit is that the getrect routine doesn't need to fetch the bboxes
of the shapes inside the viewport. It just needs the svg width, height, x,
y, and parent transform. It's also possible to add an optimized branch if
there is no parent transform specified.

One drawback is that the HTML5 method getBBox() includes the clipped content
in its calculation. That means the user will get scrollbars even though the
[draw svg] is clipping the contents which cause the scrollbars. Hopefully we
can just work around this by adding a "scrollbars" method for canvases to
just turn them off altogether.

Code for an experimental [draw array] is also added here. Combining it with
the viewport of [draw svg] will make it possible to interact with arrays of
widgets without the performance penalty normally associated with mouse
motion on Pd canvases.

Also:
* fixes some display bugs in certain cases for [draw image] and
[draw sprite]
* adds a default png for both [draw image] and [draw sprite] if none are
given

This patch makes it possible to have multiple running instances of Purr Data,
and to leverage that interface from Pd-l2ork to get [pd~] working on all
platforms

Some difficulties required revisions to [pd~] in Pd Vanilla's code
* use quoted path args for Windows when starting the [pd~] subprocess
* replace sys_havegui() from Pd Vanilla's recent "threadening" with sys_nogui

The "-unique" flag requires the OSX app bundle to be able to start by calling
the binary in Pd-l2ork.app/Contents/Resources/app.nw/bin/pd-l2ork. This should
also make it possible to run Purr Data on OSX both by using an app bundle
(where the GUI starts the Pd process) and by installing and running from the
command line (where the Pd process starts the GUI)

The "--user-data-dir" flag sets the directory for various nw.js state used
by a running instance. To prevent races nw.js requires that each running
instance have its own directory that is not shared with any other instances.

Setting the value for this flag has two important potential side-effects:

1. If you have a running instance and try to start an additional instance
   that has the same data directory, nw.js will just send an "open" event
   the first running instance and exit the 2nd instance gracefully. This is
   the default nw.js behavior and the default Pd-l2ork behavior as well.
2. If you have a running instance and try to start an additional instance
   with a different data directory than the first, you'll get a fully
   functional 2nd instance of nw.js running. This is the behavior we desire
   when the user runs pd-l2ork with the "-unique" flag.

In each case we set the data dir explicitly in order to get consistent
positional command line arguments in the GUI.

In normal mode Purr Data will use the default data dir as specified in the
nw.js docs for the particular platorm. Since the same default dir is used
each time Purr Data is run, there will be only one data dir that gets created.

When run with the "-unique" flag, Purr Data will append "-$portno" to the
default data directory, where $portno is the port Pd chose to communicate
with the GUI. That ensures that each separate instance of Purr Data can
get a unique data dir.

A side effect is that users may end up generating multiple data dirs that
don't get deleted on exit. But Pd starts searching for available ports in the
same range each time, and unless the user tries to run hundreds of instances
at once this directory spam shouldn't be too big of a problem.