Blender Git Loki

Prevent copying too much in the Rigid Body simulation

To prevent the pointcache from being copied-on-write too (and requiring
copying back), the cache is now shared between the original and
evaluated scenes. Reading from the cache is always allowed; running the
sim and writing to the cache is only allowed when the depsgraph is
active.

Some pointers have moved from RigidBodyWorld (RBO) to
RigidBodyWorldShared (RBOS). writefile.c copies some pointers back from
RBOS to RBO so that the file can still be opened on older Blenders
without crashing on a segfault.

The RigidBodyWorldShared struct is written to the blend file, because it
refers to the PointCache ID block.

The RigidObjectShared struct is runtime-only, and thus not saved to the
blend file.

An RNA getter-function is used to hide the new 'shared' pointer. As a
result the Python API hasn't changed.

Reviewed by: campbellbarton

Differential Revision: https://developer.blender.org/D3508