Custom allocators don't support user-provided state

[rkevingibson]

I appreciate the ability to provide ngf_allocation_callbacks, but one problem is that the callbacks don't take a void* userdata parameter, requiring any custom allocators to rely on global state. I'm happy to do the changes and submit a PR but wanted to see if there's interest first. It seems like a fairly small change, since all allocations seem to go through macros NGFI_ALLOC/NGFI_FREE, so the change would be:

• Adding a void* userdata field to ngf_allocation_callbacks
• Changing the signatures of allocate and free in ngf_allocation_callbacks to take a void* as a final argument.
• Changing the NGFI_ALLOC/FREE macros to pass in the userdata pointer.

[nicebyte]

Sure, that would be a nice improvement. Feel free to submit a PR. Thank you!