GUI:

- almost all useful features are missing ;)

VIEWER:

- board updates should somehow be done incrementally or partly-incrementally,
  as they take the majority of the time when intelligent updates are used.
  (compute all boards when scrolling only?)

- shadow drawing is broken for incremental updates... drawn twice with alpha(?)

- various off-by-one (or off-by-half) errors ("edgy" stones)