[API] add chunk writer
Submitted by Mark Nauwelaerts
Bytewriting typically involves nested structures with sizes recorded in the bytestream (e.g. avi chunks, qt atoms, ebml elements, jpeg markers, etc). Additional proposed API aids in tracking nested structures positions and sizes,
which could be used in any of the above cases, rather than coming up with yet another home made on in each case separately.
Evidently, some variations on the proposed are possible, e.g. using a separate (inherited) type for this (GstByteWriterStack or so).