When not %NULL, @offset will contain the offset of the data in the
first memory block in @buffer and @maxsize will contain the sum of
the size and @offset and the amount of extra padding on the last
memory block. @offset and @maxsize can be used to resize the
buffer memory blocks with gst_buffer_resize().
Get the total size of the memory blocks in @b.
When not %NULL, @offset will contain the offset of the data in the first memory block in @buffer and @maxsize will contain the sum of the size and @offset and the amount of extra padding on the last memory block. @offset and @maxsize can be used to resize the buffer memory blocks with gst_buffer_resize().