configure.ac: Remove warning when building against a system version of libav

Nowadays libav has proper releases and API/ABI guarantees.
We're not living in last decade.
libav: Update to libav master (libav 10 API)
avvidenc: Port to av_encode_video2()

av_encode_video() is deprecated and an ugly API anyway.
avdemux: Stop using r_frame_rate field

It was removed and quite meaningless anyway.
Use the average framerate instead.
avauddec: Stop using AVCODEC_MAX_AUDIO_FRAME_SIZE

And properly use the have_data variable. It's a boolean.
avviddec rebase
avenc: Stop using the removed inter_threshold field
avcfg: Update config parameters a bit to fix compilation

This needs a major rework and should use the new configuration
interface where useful.
av: Update for CodecID -> AVCodecID related changes
avcodecmap: fix regression with wmv3 video in WMVA format

The VC-1 decoder should handle that. Before avdec_wmv3 handled
it, but then we added format=WMV3 to its sink pad template
caps, at which point nothing handled WMVA any longer.

https://bugzilla.gnome.org/show_bug.cgi?id=697665
libav: Update to v10_alpha1 tag
av: Enable HEVC / h265 support
avauddec: Give MP3 decoder a MARGINAL rank too

There's no reason why this decoder should not be autoplugged
avviddec: release buffers when not direct rendering

New libav will not call the release_buffer callback anymore when
avcodec_default_get_buffer() is called from get_buffer. Releasing of the
memory in a picture should now be done by registering a callback to the
avbuffer objects in the picture. There is some compatibility code to
wrap the memory we provide in get_buffer in avbuffer with a callback to
release_buffer but that is not done when avcodec_default_get_buffer()
is called.

Work around this by adding a dummy avbuffer object to the picture that
will release the frame.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=721077
avviddec: set output chroma-site

Use libav provided chroma-site and place it on the output caps.
gst-libav: fix context leaks

A AVCodecContext needs cleaning up before being freed.
Go through all of the allocations/setups to ensure none of them
can leak a context or its contents.
avviddec: Mark frames as corrupted if libav tells us they are

This is currently only implemented by the h264 codec.

https://bugzilla.gnome.org/show_bug.cgi?id=722290
avauddec: Mark frames as corrupted if libav tells us they are

This is currently only implemented by the h264 codec and no audio codec.

https://bugzilla.gnome.org/show_bug.cgi?id=722290
avviddec: Add output-corrupt property

The output-corrupt property will set the CODEC_FLAG_OUTPUT_CORRUPT flag
in the codec context. The user can now decide whether libav outputs
corrupt frames or not.

Previous libav versions had this flag always set.

https://bugzilla.gnome.org/show_bug.cgi?id=722453
libav: Update to version 10_alpha2