The field still remains in MpegEncContext because it is used by the
mpeg4 decoder.

It is only used in one place immediately after it is set.

That field is never set for encoding.

The field remains in MpegEncContext for use in the encoder.

The line setting it was mistakenly removed in
ecf442a5

This supplements the deprecated request_channels-based control of XCh decoding.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

The check for (prim_channels > 2) before calling dca_downmix made these
cases unreachable, but now 2.1 layouts will go through the downmix code.
Having dual mono, Lt/Rt and sum-difference layouts print errors when
regular Stereo doesn't seems pointless.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Embedded downmix coefficients can use this.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Previous coefficients were producing a non-normalized, incorrect Lt/Rt downmix.

The new coefficients produce a standard Lo/Ro downmix with 3dB attenuation.

This should match the typical default coefficients used by DTS encoders.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

As per ETSI TS 102 114 V1.4.1 specification.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

It was based on an old, seemingly incorrect specification, so default
coefficients were always used anyway.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Easier to read, modify, and avoids relying on an outdated table.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

The 7-bit codes previously used are absent from the ETSI 102 114 V1.4.1 spec.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

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

These parts are scheduled for removal on the next version bump.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

Fixes out of array accesses and integer overflows.

This will check if -wN '..@*' is available and fall back on -x if not;
when none are available, do not run strip at all to prevent removing
functions that might be actually needed.

Call ff_mpeg4_decode_picture_header() only when the decoder has the
MPEG4 codec id, not based on a vaguely related value of h263_pred.

Fixes invalid writes.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org