ac3: set default matrix encoding modes in parse_frame_header.

Deduplicates some code.
(e)ac3: parse and store the Lt/Rt and LFE mix levels.
(e)ac3: clip surround mix level indexes.

Values 0-2 are reserved, 3 (-1.5dB) should be used instead.
eac3: cosmetics, re-indent.
Merge commit '1f604f96ea70503caa642f68a85be6074a5b3f46'

* commit '1f604f96ea70503caa642f68a85be6074a5b3f46':
  ac3: set default matrix encoding modes in parse_frame_header.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'c229f571fd3c7d7b567c27c87b2bbcdaee1b0e9f'

* commit 'c229f571fd3c7d7b567c27c87b2bbcdaee1b0e9f':
  (e)ac3: parse and store the Lt/Rt and LFE mix levels.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'ade75fb811500f3e3f284737f123938d83be728f'

* commit 'ade75fb811500f3e3f284737f123938d83be728f':
  (e)ac3: clip surround mix level indexes.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  eac3: cosmetics, re-indent.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
lavf: add avformat_get_mov_video_tags() and avformat_get_mov_audio_tags()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>