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)

*added outlet to [draw] to notify for events
*added svg-style event mousedown (more to come)
*added a "drag" event for convenience (similar to curve_motion)
*fixed default strokelinejoin per svg spec
*event "bubbling" so that the closest shape to front gets the notification
*added "bbox" method to turn off bbox calculation (similar to curve's -n flag)
*added a "pointerevents" method per svg-spec
*fixed skewx and skewy transforms to use degrees instead of radians

*reversed "fix" to a regression made in a previous commit (see commit 28b0256f) and removed the old commit that caused the regression in the first place.

*added getscroll after redrawing gop and scalars that have been changed via properties dialog in case their xy positions changed