Implement NiBillboardNode flags (#3602)

Closes #3602 (closed)

This replaces the callback with a customized Frankenstein's monster inspired by osg::AutoTransform and based on NifOsg::MatrixTransform. Deriving the class from MatrixTransform is necessary to theoretically support animations without suffering from headaches and doing the cursed "transform + cool node" chaining. Rotate about Z rotates around the object's local Z, rigid face camera mode is inverted on camera Z compared to master but looks the same, while Always Face Camera... Well, you can read about it in the issue, I can't really explain what it does

osg::AutoTransform does not have modes that are relevant to us after all.

Examples B and C in Kurp's test level are ROTATE_ABOUT_UP demonstrations. Note how the trees rotate around a single axis. Example D and later are ALWAYS_FACE_CAMERA which was implemented as RIGID_FACE_CAMERA at the time this was recorded, i.e. the classic behavior everyone is familiar with. In master, B and C look the same as D.

This had minimal testing from my side and was submitted to get early opinions, need to get/produce more examples (particularly animated examples). I think the fundamentals are sound and this is very close.

• I'm unsure how to handle paging or how it's currently handled in general. Current resolution: use the neutral pose
• This is an existing issue, but collision shapes should maybe be animated (verify). Resolution: naive approach causes problems, let's say this is a known issue.
• Morrowind's Rigid Face Camera facing direction seems to be inverted. Resolution: it's inconsistent about this, in some cases master orientation is fine, so keeping master behavior for this mode isn't the end of the world. I was accidentally testing the other face camera mode, it is consistent after all.
• Need to finish and push Always Face Camera