buffer: add api to get the current memory offset
Also return the offset in a GstMemory block with the get_sizes() method. This allows us to figure out how much prefix there is unused. Change the resize function so that a negative offset can be given. This would make it possible to resize the buffer so that the prefix becomes available. Add gst_buffer_get_sizes() to return the offset and maxsize as well as the size. Also change the buffer resize method so that we can specify a negative offset to remove prefix bytes.
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