rlottie: refactor to use a local arena allocator for shadow tree creation

rlottie: refactor to use a local arena allocator for shadow tree creation

This usecase is best suitable for local arena allocator as we have a fixed number
of objects to create and they all gets deleted at once. so by keeping a allocator
in the composition and use it to create all the objects using it will basically
flattened the memory layout of the shadow tree as all the memory comes from the one
memory allocator pool.
This improves the bencmark performance by 5%.