crasher fix. Match the raw access protection with the intended usage.

The actual raw access usage is to consume the packet data bu
reading it in chunks. When the reading (invoked by the decoding loop)
is done, the data changed has to be reset.
Otherwise, everything just crashes and burns. :)
complete AVFrame structure.

Known problems:
- (to be verified) not part of public API/ABI, thus special care
  needs to be taken for version bumps.
- we need harder and harder a tool to automatically extract
  (part of) structs from the headers, much like make_enum.
more AVFrame accessors.
Pull from HG.