Remove the user_data from the alloc vmethod. Subclasses that implement a new
alloc function can also implement their own vmethod to pass extra arguments. We
can then also require that custom allocators implement an alloc function so that
gst_allocator_alloc() always works.
Make GstAllocator a GstObject instead of a GstMiniObject, like bufferpool.
Make a new gstallocator.c file. Make a GstAllocator subclass for the default
allocator.
Expose the GstAllocation structure and provide an _init function. This makes it
easier to make 'subclasses' of the allocator that contain more info.
It also allows us to expose the flags on the allocator miniobject.
Make a flag to note that the allocator uses a custom alloc function.
