buffers (few, if any, elements implement this as of May 2009)
bytes
the default format of the pad/element. This can be
samples for raw audio, frames/fields for raw video (some, but not all,
elements support this; use GST_FORMAT_TIME if you don't have a good
reason to query for samples/frames)
percentage of stream (few, if any, elements implement this as of May 2009)
time in nanoseconds
undefined format
Standard predefined formats