1. Allocators don't implement memory free() methods since all the memories will
implement dispose() returning FALSE
2. Memory/miniobject dispose() will act as memory release, enqueueing the
release memory
3. A new allocator's method prepare_buffer() which queries the released memory
queue and will add the requiered memories to the buffer.
4. Allocators added a GCond to synchronize dispose() and prepare_buffer()
5. A new allocator's method flush() which will free for real the memories.
While the bufferpool will
1. Remove all the memories at reset_buffer()
2. Implement acquire_buffer() calling allocator's prepare_buffer()
3. Implement flush_start() calling allocator's flush()
4. start() is disabled since it pre-allocs buffers but also calls
our reset_buffer() which will drop the memories and later the
buffers are ditched, something we don't want. This approach avoids
buffer pre-allocation.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1626>
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