The issue was introduced in rB4b685e1 and it appears some crazy area still
accesses particles for render after deleting it's render data, which broke
viewport/render behavior.
This commit restores previous G.is_rendering logic and adds corresponding
checks to cache construction, so counting is all consistent.
Goes to the TODO list to either replace G.is_rendering with eval_ctx or to
make it so psys->renderdata always exists during render sync.
c523e82e31