Support allocators

[p0nce]

• do people want allocators just for image data or also all the various things, like metadata?
• Is allocator a global? If no, images need to retain their allocation method. If that is so, we need a user pointer to give to the allocator.
  The problem of a global is that then the library needs initialization, which it currently doesn't.
• At this point it would be handy to store some thing like that as metadata (for example: initial allocation method).

[p0nce]

Discussion here => https://forum.dlang.org/post/ncjslpwwtzywyqblhtyk@forum.dlang.org

[p0nce]

Interesting design here: https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VkAllocationCallbacks.html