The current API allowed to have a buffer for each primitive in a
renderable. We instead restrict the API so that there is a single
MorphTargetBuffer for the whole renderable, shared by all primitives.
The buffer can be shared thanks to the "offset" parameter on
setMorphTargetBufferAt().
Also
- fix FMorphTargetBuffer::updateDataAt()
- add support for the "offset" parameter of setMorphTargetBufferAt()