don't simplify trzaffic network quite so aggressively
move line segment rendering to cached ImageCanvas layer
re-factor animated car handler into a separate function
re-factor visible link finder into common function

The animated cars and coloured links now share code.
minimum link size when simplified is 5 pixels
tidy up car animation drawing
zoom to full extent on load
don't try rendering cars on tiny links
add crude stats pane
add data interpolation
add data interpolation and age filtering
don't colour invalid (no data) links
stub in a scale