lavf: add AVFormatContext/AVStream fields for signaling to the user when events happen.

The only flags, for now, indicate if metadata was updated and are set after each call to
av_read_frame(). This comes with the caveat that, on stream start, it might not be set properly
as packets might be buffered in AVFormatContext.packet_buffer before being given to the user
in av_read_frame().

Signed-off-by: Anton Khirnov <anton@khirnov.net>
AVOption: add support for dictionary types.

In order to support metadata being set as an option, it's necessary to be able
to set dictionaries as values.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
http: export icecast metadata as an option with name "metadata".

Signed-off-by: Anton Khirnov <anton@khirnov.net>
http: enable icy metadata by default.

It won't hurt servers that don't care about the header,
and those that do will include it by default.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Set protocol-level metadata in AVFormatContext any time a packet is read.

If any option named "metadata" is set inside the context, it is pulled up to
the context and then the option is cleared.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Expose metadata found in onCuePoint events in .flv files.

Currently, only onMetaData is used, but some providers (wrongly)
put metadata into onCuePoint events, and it's still nice to be
able to use that data.

onCuePoint events also present metadata slightly differently than
onMetaData events: all metadata is found inside an object called
"parameters". In order to extract this metadata, it's easiest to
recurse through the object tree and pull out anything found in
child objects and put it in the top-level metadata.

Reference: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/2/help.html?content=00001404.html

Signed-off-by: Anton Khirnov <anton@khirnov.net>
flvdec: update AVFormatContext.event_flags with METADATA_UPDATED whenever metadata changes.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
nutdec: update AVFormatContext.event_flags with STREAM_/METADATA_UPDATED whenever metadata changes.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
mov: update AVFormatContext.event_flags with METADATA_UPDATED whenever metadata changes.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
ogg: update event_flags with STREAM_/METADATA_UPDATED whenever metadata changes.

Originally, AVFormatContext and a metadata dict were provided to ff_vorbis_comment(),
but this presented issues if an AVStream was being updated or the metadata on
AVFormatContext wasn't actually being updated. To remedy this, ff_vorbis_stream_comment()
explicitly updates a stream's metadata and sets any necessary flags.

ff_vorbis_comment() does not modify any flags, and any calls to it that update
AVFormatContext's metadata (just a single call) must also update
AVFormatContext.event_flags after detecting any metadata changes to the provided
dictionary, as signaled by a positive return value.

Signed-off-by: Anton Khirnov <anton@khirnov.net>