the old debug_assert always ran and tested the expression, which slows
down release builds. wrapping them in #ifndef NDEBUG is clumsy. the new
ASSERT behaves like assert and ENSURE like the old debug_assert. Let's
change any time-critical but not-super-important ENSURE to ASSERT to
speed up release builds. (already done in bits.h and unique_range.h)
This was SVN commit r9362.
Support asynchronous path queries.
Allow escaping when stuck in impassable terrain tiles.
Split Update message in multiple phases, to cope with ordering
requirements.
Support automatic walk/run animation switching.
This was SVN commit r8058.
