EMI: Implement head tracking.
EMI: Implement head limits.
EMI: Fixed Actor::getWorldPosition calculating the world transform incorrectly for attached actors.
EMI: Continuously update the look at vector when looking at an actor.
Merge pull request #947 from Akz-/emi-head-rebase2

EMI: Implement head tracking
GRIM: Set shadow userdata to nullptr when done with it.
GRIM: Implement a dim shader
GRIM: Implement GfxOpenGLS::dimRegion