Remove Memory<T>.ToArray() calls from Scene class

[an-prata]

Removes Memory<T>.ToArray() calls from the Scene class and replaces the use of IMemoryOwner with GCHandle instances. vertexHandle and elementHandle are now assigned values in construction while the class still has access to the vertexData and elementData arrays.

To do this vertexMemory and elementMemory are now constructed with the aforementioned arrays rather than getting an IMemoryOwner from MemoryPool<T>.Shared.Rent(), then copying the array to its IMemoryOwner.Memory property.