Columbus view cleanup

No need for this to make b16 today.

This cleans up Columbus view morph time handling.  It introduces a new GLSL uniform, `czm_morphTime` and removes the primitive-specific morph times, whose original intention were to allow primitives to morph at different rates.  Later, we can still achieve a similar effect by allowing each primitive to define a function that maps the scene's morph time to a primitive-specific one.

This does not fix any Columbus view or morph issues; it only cleans up code to make generic batching easier.