AM_EXT_CENTERLINE modifies existing center lines by extending or shrinking them.
+---------------<----------------+
| |
| |
---->(AM_EXT_CENTERLINE)---+---|center line|---|position|---+----->
This command has no options. Simply indicate a center line and then a point for
it to extend or shrink to.
Use this action to extend or shrink branches of existing center lines.
AM_CREATE_CENTERLINE terminate action,
AM_ROT_CENTERLINE terminate action,
AM_ROT_CENTERLINE_BY_ANGLE terminate action