Commit rB709ca0ece changed how rotation was handled for particles so
that unless actual rotation physics is enabled, there is no rotation.
However it only updated ptcache_particle_read, forgetting to change
exactly the same code in ptcache_particle_interpolate.

This means that for subframes old code that computes a rotation from
velocity is used, resulting in completely different rotation than for
integer frames. This causes rotational motion blur by itself, and also
mangles motion blur paths of child particles.