Blender Git Loki

Particles: Fix for missing particles in render if they're disabled for viewport

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.