Faces truncation bug

[BruceSherwood]

If you truncate faces.pos, the color and normal arrays are also truncated,
but incorrectly. As you can see in the following example, the pos array
loses its last 3 elements, but the color and normal array lose their first
3 elements.

from visual import *
p = [(-1,0,0), (1,0,0), (0,1,0), (1,0,0), (-1,0,0), (0,-1,0) ]
c = [color.red, color.orange, color.yellow, color.green, color.cyan, color.blue]
n = [(0,0,0.1), (0,0,0.2), (0,0,0.3), (0,0,0.4), (0,0,0.5), (0,0,0.6)]
f = faces(pos=p, color=c, normal=n)
print('------------------')
print(f.pos)
print('------------------')
print(f.color)
print('------------------')
print(f.normal)
print('******************')
scene.mouse.getclick()
f.pos = f.pos[:-3]
print(f.pos)
print('------------------')
print(f.color)
print('------------------')
print(f.normal)

[BruceSherwood]

The same problem applies to curve, and there doesn't seem to be a way to do anything about it. The statement f.pos = f.pos[:-3] could just as well be f.pos = array([1,2,3], [4,5,6], [7,8,9]); in other words, we have no idea what portion of the color or normal arrays to use in the truncated object. A possible approach would be to create a new method: f.truncate(i,j) would replace the current pos, color, and (in the case of faces) normal arrays with pos[i:j], color[i:j], and normal[i:j].