Add track description flag to control whether missing values can be reconstructed through interpolation

[nfrechette]

While most tracks contain samples that can be safely interpolated, it is not always the case. This is where per track rounding comes in handy. For example, some scalar tracks might represent integral values that are discreet and not meant to be continuous. While values may not be perfectly preserved due to the lossy nature of ACL compression, rounding those values should work as intended.

This works fine until key frames are removed for database streaming. If a boundary key frame is removed, its value cannot be reconstructed through floor/ceil/nearest in all cases. It becomes necessary for these tracks to retain boundary key frames: key frames with a non-zero velocity/derivative.