make_trail bug

There's a bug in the make_trail option for leaving a trail when an
object moves. Turns out that you have to update pos for the trail to
be generated; updating just pos.x doesn't work:

ball = sphere(make_trail = True)
x = .01
ball.pos.x += x # does not leave a trail
ball.pos += vector(x,0,0) # this works correctly

Bruce Sherwood

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