#57 slicing curve pos and color together

[Bruce Sherwood]

You can delete elements from curve.pos or curve.color
by slicing these lists, but slicing one of the lists can
get colors and points out of sync with each other.
What is needed is a way to delete corresponding
elements from both lists. Dave Scherer suggests a
syntax of the form

c = curve(.....)
del c[3:7]

This problem comes up for example when leaving a
trail behind a moving object, with colors along the trail
indicating something such as speed, and wanting to
keep the trail a finite length by deleting the oldest
points.