Transform memory management

[TzuChieh]

Transforms do not occupy memory as much as primitives (unless meshes are created with one-transform-per-primitive basis); we should just pass data around rather than raw pointers.

[TzuChieh]

Objection. See Swallowing the elephant (part 2). Apart from the use of std::map, transforms do use a fair amount of memory by themselves. Currently our cooking system can easily map identical transforms. Sharing identical transforms would likely to benefit Photon as well.

• [ ] Share identical transforms during cook