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.
Reviewers: sergey, lukastoenne
Maniphest Tasks: T45936
Differential Revision: https://developer.blender.org/D2124
c1fd97f15d