Consider introducing a simple way to get a MemoryAllocator from the .NET Memory manager types

[john-h-k]

The System.Buffers namespace contains the ArrayPool<T> and MemoryPool<T> types which are effectively the standard library allocator types. Many custom allocators are derived from these 2 types. Currently, the ImageSharp allocators don't seem to have a way get one based off one of these types. We have ArrayPoolMemoryAllocator, which is great, but it doesn't seem to offer a way to create this from a custom ArrayPool<T>, which can be frustrating

[antonfirsov]

It's relatively easy to extend ArrayPoolMemoryAllocator, just needs a new constructor. I'm fine reviewing a PR dealing with this. Please note that ArrayPoolMemoryAllocator actually uses 2 array pools with different configurations: one for small, one for large buffers.

For MemoryPool we need a new MemoryAllocator type, so it's more complicated, needs analysis + design work + discussions.