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

commit: 418c7380b066db065dcc8a3315f4a6ca61f7bf1c

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

cherry-pick Albert Gräf's branch: Comment out a bunch of debugging statements which add a lot of noise to the console.

port c6771297b6f03887034c5f121d3dbf701623ab87 from Pd-l2ork: the big Vanilla forward port won't compile without this commit

bind/unbind scalar symbol in scalar_vis instead of scalar_new
This way scalar events continue to work if the user happens to
change the [struct] arguments on us.

code for this is ugly (i.e., repetitive) and can probably be easily abstracted/refactored...

Make it possible to just send new attributes to GUI for [drawpolygon] and friends when a field value changes
(This performs better than erasing and redrawing the scalar in the GUI)

do a proper loadbang for canvas fields (needs more testing)