use canvas_getrootfor instead of glist_getcanvas, guarantee editor gets created when reloading abstractions

remove redundant call in g_mycanvas.c and filter repeated label updates if
the label hasn't changed as in Pd Vanilla

However, we still need to figure out if we want to supress gui_canvas_getscroll
when suppressing the scrollbars. Currently it still gets called, which will
adjust the viewport to make objects at negative indices visible. I think
instead we want to anchor the viewport to 0,0 at top left and supress those
calls to get a nice speedup in the GUI.

commit: 3f92d75115d8e8a2ab5a8234be0326b6391ac378

(tcl errors on closing a patch in which a scalar was getting a deferred
update, usually I think because of an array).

commit: 418c7380b066db065dcc8a3315f4a6ca61f7bf1c

just leave the title unchanged you don't get "save as" action again on
next save

commit: 470032130e2b0135b97151d719c4706d68f3c380

We ignore the reversion and workaround in a later vanilla commit for an
OSX tk issue that doesn't apply to nw.js

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