gschem: Fix rendering artifacts when object creation is cancelled

gschem: Fix rendering artifacts when object creation is cancelled

Object creation cancelled due to dragging out an invalid sized object
(zero radius circle, box or picture with zero width or height), left
an impression of the rubber-band lines on the screen.

Cancelling object creation with the escape key does not cause this
effect, as it triggers a state change which includes a complete redraw
of the schematic page.

TODO: Actually write the code!