A dimension to a line, or between two parallel lines, drew its extension lines from the
measured end point and its foot on the other line, so they ran the whole length of the
lines to wherever that end happened to be. They now start from the nearest point of each
line to the dimension line, and disappear where the dimension line already crosses it.
Fixes#1361
Co-authored-by: Claude Opus 5.5 <[email protected]>