Fix default size of circle

Previous default size was radius 1, not diameter 1. This made the circle
double the size of other primitives.
Fix for strange position behaviour

Somewhere along the line I broke position/rotation/scale interactions by
returning the vectors themselves rather than copies...
Two small bug fixes on main view

1) Typing in the script editor kept triggering rerender on the preview
window. Forgot to stick the timer trigger in the conditional...

2) Second bug... the preview window got messed up after returning from
the final render window. Now the system reinitialises the raytracer on
return from the final render window
Hook up buffer setting changes post render

Exposure and factor changes update image even after rendering has
stopped