memory: use allocators to allocate memory
Rename the GstMemoryImpl to GstMemoryAllocator because that's really what it is. Add an alloc vmethod to the allocator members. Improve registration of allocators. Add methods to get and set the default allocator Always use an allocator to allocate memory, use the default allocator when NULL is passed. Add user_data to the allocator Info so that we can pass extra info to the allocator new method.
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