buffer: add api to get the current memory offset

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.