lavr: mix front center channel as indicated in the ATSC A/52 specification.

When mixing 3 front channels into 2, the center channel is mixed into left and right with the center mix level, not -3dB.
lavr: mix Dolby Surround as intended in the ATSC A/52 specification.

The specification indicates -3dB, but some streams carry additional mix levels intended specifically for Lt/Rt output, so use the surround mix level instead.

Either way, applying both -3dB and the surround mix level is incorrect.
lavr: use the same weight for side and back channels when mixing one set into the other.

This matches the default embedded downmix coefficients of several encoders.