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.

make [draw] arguments more closely follow the svg spec
* [draw circle 20 1 2]: radius=20, xcenter=1, ycenter=2
* [draw rect 5 10 20 40]: width=5, height=10, xoffset=20, yoffset=40
* [draw ellipse 5 10 20 40]: xradius=5, yradius=10, xcenter=20, ycenter=40

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

add a flag to drawsymbol/number visfn to allow updating existing objects instead of having to erase/redraw them

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)

change fielddesc_getfloat to fielddesc_getcoord so that the (admittedly arcane) scaling/quanta syntax can be used with [draw] methods where it makes sense

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

from pd-l2ork: f3c4044dc6a28185488f365be8a6ebd61fba6315