1.1-based RT-safe fix for race condition causing #1662

[Fastigium]

I couldn't find a way to rebase my existing pull request, so here's another one, this time against stable-1.1. Tested and works for me. Original description:

Ok, I fixed things the atomic way. By setting and reading m_dependenciesMet simultaneously atomically, the race condition is eliminated without adding a lock. It even gives me less clicks and pops than the lock fix I proposed earlier :) #1662

[tresf]

git cherry-pick is usually what we use, but that tends to only work if the files are still relatively similar. No worries. Thanks for the fix!