when an edge is dirty, mark all outputs dirty

when an edge is dirty, mark all outputs dirty

The logic before was like:
  for each output:
    if edge_dirty or output.dirty:
      output.dirty = true
      edge_dirty = true

This was wrong in the case where the second output would case the edge
to be dirty; we needed to go back and mark the first output dirty as
well.  Fixed by taking two passes: one to compute the dirty state,
then a latter sweep to mark all outputs.

Fixes issue 148.