The geometry of a track made anew, ten times a second while it's
dragged, only forgot the chunks it had: the bank kept them all, so the
buffer of the section grew by the whole track and its neighbours each
time and was sent to the gpu in full again. Ten rebuilds of a test
scenery took the bank from 2 to 29 MB; a long session would run the
card out of memory. The old chunks are erased from the bank now, but
for those of a moving nose, laid again in place, and a moved traction
wire drops its old one too.
A chunk swapped for one without the extra vertex data kept the flag of
having it and the upload read from an empty array; the buffer is made
again when that changes.
