Previously, adding a midpoint to an invalidly doubled-back
segment (aba) resulted in a self-intersection with an invalid
consecutive node (accba). Now a self-intersection is still
produced, but with only one c node (acba).
Refs #1296
Due to differenced redraw, midpoint.ways was getting stale,
so sometimes dragging a midpoint didn't add the new node to
all ways. Better to calculate participating ways when needed.
This also simplifies the code for adding a midpoint via double
click. It does make filtering midpoints on redraw a bit more
expensive, but a cursory profile didn't show it being a problem.
Fixes#763.