lavc: remove the locking code in avcodec_close()

This function should not modify any global state, so there should be no
reason for any locking.
lavc: add avcodec_free_context().

Right now, the caller has to manually manage some allocated
AVCodecContext fields, like extradata or subtitle_header. This is
fragile and prone to leaks, especially if we want to add more such
fields in the future.

The only reason for this behaviour is so that the AVStream codec context
can be reused for decoding. Such reuse is discouraged anyway, so this
commit is the first step to deprecating it.
4xm: allocate extradata properly.

Pad it with the required amount of zeros, check for malloc failure.
Merge commit 'e1b66778b6ee82a192b5895e23c4e135f7269326'

* commit 'e1b66778b6ee82a192b5895e23c4e135f7269326':
  lavc: remove the locking code in avcodec_close()

Conflicts:
	libavcodec/utils.c

Note, if someone knows of a external codec lib/API, hwaccel or other that requires
locking on its close/free side, please contact ffmpeg-devel.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'fd056029f45a9f6d213d9fce8165632042511d4f'

* commit 'fd056029f45a9f6d213d9fce8165632042511d4f':
  lavc: add avcodec_free_context().

Conflicts:
	doc/APIchanges
	libavcodec/version.h

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'd256ed78ffe202a4dcc8d625becffc716bfa3977'

* commit 'd256ed78ffe202a4dcc8d625becffc716bfa3977':
  4xm: allocate extradata properly.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Support raw dvdsub palette as stored on normal dvd's

This is how the palette is stored on dvd's. Currently
only xbmc passes the palette information to libavcodec
this way.
Check return value of avio_seek and avoid modifying state if it fails

The code still modifies state if the timestamp is not found. Not
sure exactly how to avoid that.
asf hacks
if av_read_packet returns AVERROR_IO, we are done. ffmpeg's codecs might or might not handle returning any completed demuxed packets correctly
added: Ticket #7187, TV Teletext support for DVB EBU Teletext streams
Don't accept mpegts PMT that isn't current
Don't reparse PMT unless it's version has changed
Read PID timestamps as well as PCR timestamps to find location in mpegts stream
Get stream durations using read_timestamp
changed: allow 4 second skew between streams in mov before attempting to seek
fixed: memleak in mpegts demuxer on some malformed (??) mpegts files with too large pes packets

at-visions sample file brokenStream.mpg
Speed up mpegts av_find_stream_info
dxva-h264 Fix dxva playback of streams that don't start with an I-Frame.
add public version of ff_read_frame_flush

We need this since we sometimes seek on the
input stream behind ffmpeg's back. After this
all data need to be flushed completely.