class OnOutputPacket

This is how the stream processor emits an output packet to the stream

processor client.

Order is significant.

The client should eventually call RecycleOutputPacket() (possibly after

switching streams multiple times), unless the buffer_lifetime_ordinal

has moved on. A stream change doesn't change which packets are busy

with the client vs. free with the server.

The relevant buffer is always the one specified in the packet's buffer_index field.

For low-level buffer types that support it, a StreamProcessor is free to

emit an output packet before the low-level buffer actually has any

usable data in the buffer, with the mechanism for signalling the

presence of data separate from the OnOutputPacket() message. For such

low-level buffer types, downstream consumers of data from the emitted

packet must participate in the low-level buffer signalling mechanism to

know when it's safe to consume the data. This is most likely to be

relevant when using a video decoder and gralloc-style buffers.

The error_ bool(s) allow (but do not require) a StreamProcessor server

to report errors that happen during an AU or between AUs.

The scope of error_detected_before starts at the end of the last

delivered output packet on this stream, or the start of stream if there

were no previous output packets on this stream. The scope ends at the

start of the output_packet.

The error_detected_before bool is separate so that discontinuities can be

indicated separately from whether the current packet is damaged.

The scope of error_detected_during is from the start to the end of this

output_packet.