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 emitters that are fully running as soon as the game starts.
Fix particle clumping when moving from off-screen to on-screen, by
computing updates more incrementally.
Fix overzealous culling of models with particle emitters.
Add particles to renderer stats.
This was SVN commit r9345.
