We don't know how long the caller will hang on to this data, so we need
to be safe by default and assume it will kept indefinitely. That means
we can't return a reference to the internal buffer, as that will get
overwritten with future messages.
We want to avoid unnecessary copying in performance critical code,
though. So allow code to explicitly ask for a shared buffer, assuming
they know the data needs to be consumed immediately.
We don't have to keep track of this much data between rects, so
restructure things to make it more simple. This allows the JPEG parsing
code to be a pure function which only depends on the input.
Pierre Ossman
pdlan