Give MeshCache a "loop" or keyable start frame option

Playing with MeshCache to make a looping physics sim, I noticed it's not possible to specify a start frame within the MeshCache. It's also not possible to loop the MeshCache.

This simple feature would provide a checkbox called "Loop" that would modulo the current frame against the MeshCache frame length, rather than clamping to the length. It may also be better to provide a keyable current frame (within the cache) property.

The use case here is to make a looping physics sim that loops over and over across the timeline in a very simple way.